Sân: Hai hình trên là sân Football, sân football nhìn chung giống sân bóng đá, kích thước tương tự nhau. Tuy nhiên, khác với sân bóng đá chiều dài tầm tầm 100m không cố định, sân football có chiều dài chính xác 100 yards (1 yard bằng khoảng 0.9114m - nếu ai không quen nghe từ yard cứ hiểu 1 yard = 1 met cũng không có gì sai) cộng thêm phần cuối sân ở hai đầu sân, mỗi bên 10 yards nữa. Tổng cộng là 120 yards từ gôn bên này đến gôn bên kia.

Ở Mỹ có hai giải football lớn, là giải chuyên nghiệp NFL (National Football League) và giải nghiệp dư (tạm gọi thế) College Football của các trường đại học với nhau. Trong bài viết này tôi tập trung vào College Football. Nhìn chung luật chơi cơ bản giống nhau, chỉ có một số khác nho nhỏ.

Số Cầu Thủ: Mỗi bên có 11 cầu thủ, khác với bóng đá là đội hình 11 cầu thủ có hậu vệ - tiền vệ - tiền đạo - thủ môn , còn ở Football 11 cầu thủ lúc ra sân thì một bên là tấn công, và một bên là phòng thủ. Tức 11 cầu thủ của bên tấn công chỉ làm nhiệm vụ tấn công, còn bên kia chỉ làm nhiệm vụ phòng thủ. Tất nhiên có tên các vị trí khác nhau, nhưng có lẽ chưa cần nói ra kỹ ở đây.

Chơi thế nào? Ban đầu một đội tấn công có bóng, và thường xuất phát ở vạch 20 yard của bên sân mình. Như ở hình 1, sân được chia làm 100 yards, và có vạch ngang cứ 10 yard một. Đội tấn công có 4 lần chơi (tôi sẽ nói rõ hơn thế nào là một lần chơi), làm thế nào để đưa bóng đi tiến về phía phần sân đội bạn được 10 yards. Nếu sau 4 lần chơi mà không tiến được đủ 10 yard, thì phải nhường bóng lại cho đội bạn. Và đội bạn sẽ thay toàn bộ cầu thủ phòng thủ ra, và thay 11 cầu thủ tấn công vào, còn đội mình làm ngược lại, thay toàn bộ 11 cầu thủ tấn công bằng 11 cầu thủ phòng thủ. Nếu trong 4 lần chơi đấy mà tiến được 10 yard trở lên, thì sẽ được thêm 4 lần chơi nữa. Cứ như vậy, nếu đội tấn công đưa bóng được đến vạch cuối sân của đối thủ thì được 6 điểm - gọi là một quả touch down- sau đấy sẽ được thêm một điểm thưởng nữa. Muốn được điểm thưởng này, phải đá bóng từ vạch 20 yards vào gôn đội bạn - thế nào là vào gôn. Chỉ đá bóng vào giữa hai cái gôn cao cao như hình 2 là được.

Thực ra có 4 lần chơi, nhưng nếu chơi hết 3 lần mà chưa tiến được 10 yard thì sẽ có hai giải pháp. 1. Nếu bóng ở gần phần sân mình, thì đội mình phải đá bóng trả lại cho đội bạn, đá càng xa về phía sân đội bạn càng tốt - và như thế chuyển quyền tấn công cho đội bạn, đội mình phòng thủ. Còn nếu không làm thế, chơi hết lần thứ 4 mà mình vẫn không di chuyển được tổng cộng hơn 10 yard, thì đội bạn sẽ lấy bóng từ vị trí sân mình. Như thế đội bạn sẽ dễ dàng ghi điểm -> dễ thua. 2. Nếu bóng ở gần phía gôn đội bạn, thì đội mình có thể chơi lần thứ 4 bằng cách đá bóng vào gôn. Nếu đá vào được thì sẽ được 3 điểm. Tất nhiên đá quả bóng bầu dục, và nặng hơn bóng đá của mình vào gôn cũng không dễ lắm.

Sau mỗi lần ghi điểm, 7 điểm (touch down) và 3 điểm (đá vào - field goal) thì đều phải trả bóng cho đội bạn, bằng cách đá bóng từ giữa sân về phía sân đội bạn. Và đến lượt đội bạn tấn công, còn đội mình phòng thủ.

Một lần chơi, cầu thủ hai đội xếp hàng tại điểm lấy bóng, và chụp đầu vào nhau. Tất cả cầu thủ hai đội không được vượt quá vạch xuất phát sang phần sân đội bạn. Giống như khi giao bóng ở bóng đá, toàn bộ cầu thủ đội mình phải ở phần sân nhà. Bóng được đưa cho cầu thủ chuyên chuyền bóng (quaterback). Cầu thủ này quyết định ném bóng cho một cầu thủ đội mình đã chạy lên phía trên, hay đưa bóng cho một cầu thủ phía sau ôm bóng

chạy. Đấy là bên đội tấn công, còn đội phòng thủ làm gì? Đội phòng thủ tìm mọi cách để chặn không cho đội tấn công đưa bóng tiến lên, bằng mọi các, xông vào ôm, vật, kéo áo, nắm tay,.. hoặc lao cả người vào cầu thủ đang chạy. Tất nhiên không được xỉa bằng chân như bóng đá của mình, hoặc đấm vào người cầu thủ đang có bóng (chủ ý làm bị thương). Và luật của nó cũng rất cụ thể, lúc nào thì được ôm chặt, vật, lúc nào chỉ được dùng tay đẩy ra. Nếu cầu thủ chuyền bóng của đội tấn công ném bóng cho đồng đội mình, nhưng bị một cầu thủ đội phòng thủ bắt được bóng, thì quả đấy gọi là turn over, và bóng lập tức được đổi cho đội bạn. Đội phòng thủ bây giờ trở thành đội tấn công, mà không cần chờ đội kia chơi hết 4 lần.

Xem ra luật chơi cũng đơn giản, không phức tạp lắm. Nhưng chi tiết thì cũng có nhiều vấn đề, tuy nhiên về cơ bản chỉ đơn giản như thế. Mỗi trận football có 4 hiệp, mỗi hiệp 15 phút, tuy nhiên thời gian thay đổi từ đội tấn công sang đội phỏng thù, phải thay toàn bộ 11 cầu thủ ra khỏi sân, nên cũng mất thời gian. Chính vì thế mỗi trận đấu thường kéo dài khoảng 4 tiếng. Số lượng trọng tài cũng đông, có một trọng tài chính và có thể có đến 4-5 trọng tài phục

A diagram showing an "I Formation" on offense and a 4-3 Formation on defense

In American football, each team has eleven players on the field at one time. Because the rules allow unlimited substitution between plays, however, the types of players on the field for each team differ depending on the situation. At the college and National Football League levels, most play only offense or only defense, with "two-way" players being a thing of the past.

Offense (Tấn công)

The offensive team or offense in football is the team that begins a play from scrimmage(thời gian ngưng bóng) in possession(sở hữu) of the ball. A play usually begins when the quarterback(tiền vệ) takes a snap (nhận bóng)from the center and then either hands off to a running back or even a receiver, passes to a receiver or a running back, runs the ball himself, spikes the ball or takes a knee.

The purpose of the spiking the ball is to stop the game clock if the offense is running out of time. The purpose of taking a knee is to allow the clock to run with minimal risk of turning the ball over. If a player runs the ball and stays in bounds, or if a player receives a pass and stays in bounds (this has the same effect as taking a knee), then the clock keeps ticking. But if a player running the ball goes out of bounds, or there is an incomplete pass, then the clock stops.

The goal of the offensive team is to earn points for the entire team. The offensive earns points by scoring a touchdown. A touchdown results in 6 points. After the offense scores a touchdown, the special teams comes onto the field to attempt an extra point. An extra point results in 1 point. The offensive team, however, can also help the team score by getting good field position for an attempt at a field goal. A field goal earns the team 3 points.

The offensive unit in football consists of a quarterback, linemen, running backs, tight ends, and wide receivers. The function of most of the linemen is to block. The offensive line consists of a center, two guards, two tackles and one or two tight ends. Backs include running backs who frequently carry the ball and receive swing passes and screens, and a fullback, who usually blocks, and occasionally carries the ball or receives a pass. The primary function of the wide receivers is to catch passes.

The makeup of the offense and how it operates is governed by the head coach or offensive coordinator's offensive philosophy.

Center (C) — the center performs the normal blocking functions of all linemen and is the player who puts the ball in play by means of the snap. On many teams the Center will be one of the "Team Captains" and depending on the amount of autonomy allowed by the team, some centers are responsible for coordinating and directing the efforts of the other members of the Offensive Line.

Offensive guard (G) — the two guards are the offensive linemen directly on either side of the center and inside the tackles. Like all interior linemen, their function is to block on both running and passing plays. On some plays, rather than blocking straight ahead, a guard will "pull" - moving around behind the other offensive linemen upon the start of the play - in order to block a player on either side of the center, in an inside running play called a "trap" or an outside running play called a "sweep."

Offensive tackle (T)—the offensive tackles play on the outer side of each guard. Their role is primarily to block on both running and passing plays. The area from one tackle to the other is an area of "close line play" in which blocks from behind, which are prohibited elsewhere on the field, are allowed. For a right-handed quarterback, the left tackle is charged with protecting the blindside, and is often faster than the other offensive linemen to stop 'speed rushers' at the Defensive End position. Like a guard, the tackle may have to "pull," on a running play, when there is a tight end on his side.

The description above of the guard and tackle positions apply only to a line that is balanced (has equal numbers of players on both sides of the player who is to snap the ball). In an unbalanced line, there may be players designated "guard" or "tackle" next to each other.Offensive linemen cannot catch the ball but on rare occasions they are allowed and sometimes even run the ball. In most circumstances, however, they do not. Except for the snap by the offensive center as each play from scrimmage starts, ordinarily the only way an offensive lineman can get the ball during a play is by picking up a fumble. On rare occasions offensive linemen legally catch passes, known as "tackle eligible"; they can do so either by reporting as an eligible receiver to the referee prior to the snap or by catching a pass which has first been deflected or otherwise touched by an eligible receiver or a defensive player. Any other touching of a forward pass by an offensive lineman will result in a penalty.

Tight end (TE) — Tight ends play on either side of, and roughly next to, the tackles. They are a mix between a blocker and a pass receiver. If an end moves away from the tackle, he is called a split end. Modern formations typically have one tight end and one split end. Many modern formations also forego tight ends and replace them with wide receivers. Sometimes a formation is referred to as having "three tight ends." This means in reality that an additional blocker (a wingback or an eighth lineman) has been substituted for a wide receiver. This would be done as in short-yardage situations where receivers are not needed.

Wide receiver (WR) — The wide receivers are speedy pass-catching specialists. However, they can be bigger if they still can get open and catch the ball. Their main job is to run pass routes and get open for a

pass, although they are occasionally called on to block. A wide receiver may line up on the line of scrimmage and be counted as one of the necessary seven players on the line in a legal formation (a split end), or he may line up at least one step behind the line of scrimmage and be counted as being in the backfield (a flanker if he is on the outside, a slot if he is not the outside receiver but is away from the tackle, or a wingback if he lines up near [usually adjacent to and just behind] the tackle). There are generally two types of wide receivers, "speed" and "possession." A speed receiver's primary function is to use his burst speed (speed from a standing start), to stretch the field (by forcing his coverage to retreat further into the back field in the hope that at the snap of the ball they [the pass coverage] will already be near where the receiver will catch the ball). The defense has to estimate where the speed receiver will move to; and may have to pull away an eighth defensive man near the line of scrimmage who would otherwise move against the quarterback. A possession receiver is generally the more sure-handed of the two types and is used to keep possession of the ball by making catches that gain first down less yardage; but he usually lacks the speed to attack a defensive backfield. Passing long (deep into the backfield) to the possession receiver is a risk because equally quick or quicker defensive backs may be able to reach this receiver deep in their territory and (if they are so skilled), legally break up his reception of the ball. Also, if a pass is not properly thrown, is tipped, is carried by the wind (in an outdoor game), or the pass route is misunderstood between the passer and the receiver, the defense may use their equal or superior speed to make a clean interception of the ball.

Fullback (FB) — Positioned behind the middle of the line, a fullback may do some running, some blocking, and some short receiving. A classic fullback is more of a power runner than a running back. Many modern formations do not use a fullback. Most plays utilizing the fullback call for him to block, generally by running the planned rushing route behind the opening his offensive line has made in the opposite defensive lineup; and taking the block of the first linebacker(s) who tries to seal the gap the offensive line has made in the defense. By doing this the Fullback creates a path for the running back to run while having the ball. The greatest yardage can be gained by the running back when the fullback blocks, but concentrates on keeping on moving downfield with the running back advancing behind him. Should this blocking protection for the runner remain intact until the rushing convoy reaches the rear of the defensive secondary (the linebackers), and with the running back still on his feet and running downfield, one of the most memorable plays in the sport can be observed--that of an open-field run for long yardage or a touchdown.

Running back (RB) — The modern term for the position formerly called "halfback" and often referred to as a "tailback." The running back carries the ball on most running plays and is also frequently used as a short-yardage receiver. Running backs, along with the wide receivers, are generally the fastest players on the offensive team. Most of them tend not to run straight ahead, preferring to make quick cutbacks to try to find holes in the defense. The running back is in fact looking for a clear path (of any shape) to the gap between the defensive secondary (the linebacker(s)) and the defensive backfield. A modern running back still on his feet in this gap can cross the distance down field before he can be tackled by the defensive secondary (he is usually much faster than they are). His small size, his speed, and his physical strength make it very difficult for defensive backs to catch him, or to legally bring him down if they do catch up to him on the run. A new formation that is becoming more popular in football has the running back lined up as the quarterback and the quarterback split out as a receiver. This formation is known as "wildcat", and the running back lines up behind the center and receives the snap of the ball. The reason running backs are injury prone and rarely have long careers in the game is either the initial hits they receive when they run directly into the defensive line; or else hits taken from the larger linebackers while on the run which do much damage, especially to the knees and ligaments (joints). "Fullback" is now regarded as a separate position from running back, with a substantially different role (especially in the NFL).

Quarterback (QB) — Typically the quarterback is positioned to take the football when the ball is snapped (handed or passed) between the center's legs. Recent usage refers imprecisely, however, to a player who is positioned behind the center at any distance, calls signals, is not the usual punter or place kick holder, and

usually takes the snap as "quarterback" regardless of exact position, because those functions have typically been performed by quarterbacks. Typical play from formations where the quarterback takes the snap proceeds by the quarterback either handing the ball off to a running back to run, throwing the ball downfield, or running personally.

Teams can vary the number of wide receivers, tight ends and running backs on the field at one time. Football rules limit the flexibility of offensive formations. Seven players must line up on the line of scrimmage, and only the two at the end are eligible to catch passes. Sometimes, offensive lineman can declare eligibility and become "tackle eligible." Jumbo Elliott and Dan Klecko are two tackles who have caught touchdowns while being tackle eligible. Typical formations include:

One running back, two tight ends and two wide receivers. Two running backs, one tight end and two wide receivers. One running back, one tight end and three wide receivers. One running back, no tight end and four wide receivers. No running backs, no tight end and five wide receivers.

Defense

The defensive team or defense is the team that begins a play from scrimmage not in possession of the ball. The object of the defensive team is to prevent the other team from scoring. Unlike many sports, defense in American football can be very aggressive, seeking to attack the opposing quarterback and to push the offense back from the line of scrimmage. The sign that the defensive goal has been accomplished is recovering possession of the football before the offensive team scores, which usually involves the offensive team punting the ball on fourth down. Other possibilities include having the ball turned over on downs, getting an interception or recovering a fumble. Also, if an offense advances down-field and into the red zone (within the 20-yard line of the defense) and is threatening to score a touchdown, the defense can consider its goal accomplished if it forces the offense to settle for a field goal (three points) rather than a touchdown (six points). Many teams operate specific defensive plays for the so called 'Red Zone' specifically to achieve this.

Unlike the offensive team, there are no formally defined defensive positions. A defensive player may line up anywhere on his side of the line of scrimmage and perform any legal action. Most sets used in football, however, include a line composed of defensive ends and defensive tackles and (behind the line) linebackers, cornerbacks, and safeties.

Defensive ends and tackles are collectively called defensive linemen, while the cornerbacks and safeties are collectively called defensive backs, or the secondary.

Defensive end (DE) — The two defensive ends play on opposite outside edges of the defensive line. Their function is to attack the passer or stop offensive runs to the outer edges of the line of scrimmage (most often referred to as "containment"). The faster of the two is usually placed on the right side of the defensive line (quarterback's left) because that is a right-handed quarterback's blind side.

Defensive tackle (DT) — Sometimes called a defensive guard, defensive tackles are side-by-side linemen who are between the defensive ends. Their function is to rush the passer (if they can get past the offensive linemen blocking them), and stop running plays directed at the middle of the line of scrimmage. A defensive tackle who lines up directly across from the ball (and therefore is almost nose-to-nose with the offense's center) is often called a nose tackle or nose guard. The nose tackle is most common in the 3-4 defense and the quarter defense. Most defensive sets have from one to two defensive tackles. Sometimes, but not often, a team will employ three defensive tackles.

Nose guard (NG) — Sometimes called a middle guard, the nose guard lines up directly opposite the offensive center, or over the center's "nose." Nose guards tend to be shorter than most other defensive linemen. They are typically very strong and their responsibility is to stop runs down the middle and draw double teams. Extremely quick nose guards are sometimes used to shoot through the offensive line before it can react. They then sack the quarterback or make a tackle shortly after a hand off. This is rare, however, because most defensive linemen are not quick enough to consistently shoot the gaps between the individual offensive linemen. "Nose guard" is often a term incorrectly applied to the defensive tackle in a 3-4 defensive scheme. This position is known as a nose tackle.

Linebacker (LB) — Linebackers play behind the defensive line and perform various duties depending on the situation, including rushing the passer, covering receivers, and defending against the run. Most defensive sets have between two and three linebackers. Linebackers are usually divided into three types: strongside (left or right outside linebacker: LOLB or ROLB); middle (MLB); and weakside (LOLB or ROLB). The strongside linebacker usually lines up across from the offense's tight end; he is usually the strongest LB because he must be able to shed lead blockers quickly enough to tackle the running back. The middle linebacker must correctly identify the offense's formations and what adjustments the entire defense must make. Because of this, the middle linebacker is nicknamed the "quarterback of the defense." The weakside linebacker is usually the most athletic or fastest linebacker because he usually must defend an open field.

Cornerback (CB) — Typically two players who primarily cover the wide receivers. Cornerbacks attempt to prevent successful quarterback passes by either swatting the airborne ball away from the receiver or by catching the pass themselves. In rushing situations, their job is to contain the rusher.

Safety (FS or SS) — The safeties are the last line of defense (farthest from the line of scrimmage) and usually help the corners with deep-pass coverage. The strong safety (SS) is usually the larger and stronger of the two, providing extra protection against run plays by standing somewhere between the free safety and the line of scrimmage. The free safety (FS) is usually the smaller and faster of the two, providing variable and extra pass coverage. Traditionally, teams have looked for safeties with reputations as hard hitters. More recently, however, teams have been looking for hybrid safeties who can do both jobs, as in a cover 2 defense, when the strong safety has a greater role to play in coverage. Safeties are also used in a variety of blitzes.

Defensive back — It is not a specific position; however, it is any position besides the line, including cornerbacks, safeties, etc., that is behind the line of scrimmage.

Nickelback and Dimeback — In certain formations one extra (a fifth) defensive back (called a nickel defense), two extra (a sixth) DBs (called a Dime package), three extra (a seventh) DBs called a Quarter, or even four extra (an eighth) DBs called a Half Dollar may be used to augment the backfield or defensive line. Nickelbacks, dimebacks, and Defensive Quarterbacks are usually used to defend pass plays with extra receivers, but they can also be used to rush quarterbacks or running backs more quickly than linemen or most linebackers can. A starting cornerback who is good at blitzing and tackling will sometimes be referred to as a nickelback to distinguish them from cornerbacks.

Typical defensive formations include:

Five defensive linemen, two linebackers and four defensive backs (the 5-2 formation) Five defensive linemen, three linebackers and three defensive backs (the 5-3 formation) Four defensive linemen, three linebackers and four defensive backs (the 4-3 formation) Four defensive linemen, four linebackers and three defensive backs (the 4-4 formation) Three defensive linemen, four linebackers and four defensive backs (the 3-4 formation) Three defensive linemen, three linebackers and five defensive backs (the 3-3-5 formation)

Four defensive linemen, two linebackers and five defensive backs (the Nickel formation) Four defensive linemen, one linebacker and six defensive backs.(the Dime formation) Three defensive linemen, one linebacker and seven defensive backs (the Quarter formation) Three defensive linemen, no linebackers and eight defensive backs (the Half Dollar formation)

"Special teams" redirects here. For the ice hockey definition of "special teams," see powerplay and short handed.

Special teams are units that are on the field during specific situations. They include a kickoff team, a kick return team, a punting team, a punt blocking and return team, a field goal and extra point team, a field goal blocking team and "hands team" used for onside kicks to prevent the kicking team from recovering a kick, usually by recovering the ball themselves. Though fewer points are scored on special teams than on offense, special teams play determines where the offense will begin each drive, and thus it has a dramatic impact on how easy or difficult it is for the offense to score.

Because these aspects of the game can be so different from general offensive and defensive play, a specific group of players is drilled in executing them. Most special teams players are second- and third-string players from other positions, but there are also specialized players on these teams, including:

Kicker (K) — Handles kickoffs and field goal attempts, and in some leagues, punts as well. Holder (H) — Usually positioned 7-8 yards from the line of scrimmage, he holds the ball for the

placekicker to kick. The holder is often a backup quarterback or a punter. Long snapper (LS) — A specialized center who snaps the ball directly to the holder or punter. All thirty-

two NFL teams have specialized players just to long snap. Kick returner (KR) — Returns kickoffs, generally is also a wide receiver or cornerback. Punter (P) — Kicks punts. In leagues other than the NFL, the kicker often doubles as the punter. Upback — A blocking back who lines up approximately 1-3 yards behind the line of scrimmage in punting

and kneel situations. His primary job is to act as a second line of defense for the punter. Upbacks can receive a direct snap in fake punt situations.

Punt returner (PR) — Returns punts. Often the same player as the kick returner, although not necessarily so.

Gunner — A player on kickoffs and punts who specializes in running down the field very quickly in an attempt to tackle the kick returner or the punt returner.

Wedge Buster — A player whose goal is to sprint down the middle of the field on kickoffs. While ideally, their goal is to reach the kick returner, their immediate goal is to disrupt the wall of blockers (the wedge) on kickoffs, preventing the returner from having a lane in which to get a substantial return. Being a wedge buster is a very dangerous position since he may often be running at full speed when coming into contact with a blocker. This role has changed in the wake of the NFL largely banning wedges.

Object of the game

The object of the game is to score more points than the opponent during the alloted time. Points are scored when a team has control of the ball but can also be scored when they do not. The team with the ball (the offense) has 4 tries (downs) to advance at least 10 yards, and can score points once they reach or near the opposite end of the field, called the end zone. If the offense does not advance at least 10 yards during their 4 downs, the team without the ball (the defense) regains control of the ball (called turnover on downs).

On offense, points are scored by advancing the ball into the opponent's end zone for a touchdown (worth six points), or by kicking the ball from the playing field through the raised vertical posts (the goal posts) in the back of that end zone for a field goal (worth three points). After scoring a touchdown, the offense is given one additional

attempt to score. These attempts are termed point after touchdown (PAT) and are used to score 1 or 2 points as follows:

The offense may attempt a field goal kick which is worth 1 point. The offense may attempt to re-advance the ball into the opponent's end zone for a two-point conversion worth 2

points.

While on defense, the defense attempts to prevent the offense from advancing the ball and scoring. If an offensive player loses the ball during play (a fumble) or the ball is caught by a defensive player while still in the air (an interception), the defense may attempt to run into the offense's end zone for a touchdown. The defense may also score points by tackling the ball carrier in the offense's own end zone, called a safety (worth two points).

[edit] Time of play

Collegiate and professional football games are 60 minutes long, divided into four quarters of 15 minutes each. In high school football, 12 minute quarters are usually played. The clock is stopped frequently, however, so that a typical college or professional game can exceed three hours in duration. The referee controls the game clock and stops the clock after any incomplete pass or any play that ends out of bounds. In addition, each team is allowed 3 timeouts in each half that they may use at their own discretion.

The clock may also be stopped for an officials' time-out, after which, if the clock was running, it is restarted. For example: if there is a question as to whether or not a team has moved the ball far enough for a first down, the officials may use a measuring device (the chains) to determine the distance. While this measurement is taking place, the officials will signal for a stoppage of the clock. Once the measurement is finished and the ball is placed at the proper location (spotted), the referee will then signal for the clock to restart. Additional situations where officials may take a time-out are to administer a penalty or for an injured player to be removed from the field.

In addition to the game clock, a separate play clock is also used. This counts down the time the offense has to start the next play before it is assessed a penalty for delay of game (see below). This clock is typically 25 seconds from when the referee marks the ball ready for play. The NFL and NCAA use a 40-second play clock that starts immediately after the previous play ends, though for certain delays, such as penalty enforcement, the offense has 25 seconds from when the ball is marked ready. The purpose of the play clock is to ensure that the game progresses at a consistent pace, preventing unnecessary delays.

Officials also call for media time-outs, which allow time for television and radio advertising. They also stop the clock after a change of possession of the ball from one team to the other. Successful PATs (Point(s) After Touchdown), a field goal try, or a kickoff may also warrant stopping the clock. If an instant replay challenge is called during the game, the referees signal for a media time out. The referee signals these media time-outs by first using the time out signal, then extending both arms in a horizontal position.

Separating the first and second halves is halftime. Teams change ends of the field at the end of the first quarter and the end of the third quarter. In the NFL, an automatic timeout is called by the officials when there are two minutes left in both the second and the fourth quarters; this is most commonly referred to as the two-minute warning. No such warning is normally given in amateur football, though if there is no visible stadium clock, the referee will give a two-minute warning (four minutes in high school).

[edit] Overtime

If a game is tied at the end of four quarters, overtime is played.

During the regular season in the NFL, one sudden-death overtime period is played (with each team receiving two time outs) wherein the first team to score is declared the winner. If neither team scores during the 15-minute overtime, the game officially ends in a tie. Overtime follows a three-minute intermission after the end of the regulation game. Prior to start of overtime, a coin flip is performed wherein the captain of the visiting team calls the toss. The team that wins the coin flip has the option to either receive the kickoff or choose the side of the field they wish to defend. Ties are rare in the NFL; the game between Philadelphia and Cincinnati on November 16, 2008 ended in a tie, the first tie since 2002.[1]

For NFL 2010 post-season games,[2] a modified sudden-death system of determining the winner is used during overtime play. If the team (Team-A) that receives the opening kickoff in overtime scores a touchdown on their opening drive, then the game is over and they are the declared the winner. If, however, a touchdown is not scored on Team-A's opening drive, then the other team (B) is given an opportunity to possess the ball (for example, Team-A may opt to punt the ball away, or fail to make a first down on their fourth down attempt). Once the requirement for possession, or opportunity-to-possess, has been satisfied by both teams then normal sudden-death rules apply, meaning that the next team to score wins the game. Various events may occur that constitute opportunity-to-possess even though a team may not gain full possession of the ball in order to run a play. For example, a team that muffs a kickoff or punt return (i.e., drops the ball while attempting to catch it) which is then recovered by the kicking team, is considered to have had their opportunity-to-possess. If the recovering (kicking) team goes on to score, then the game is over. Similar to normal overtime, following a three-minute intermission after the end of the regulation game, a coin flip is performed wherein the captain of the visiting team calls the toss. However, in post-season overtime, multiple 15-minute overtime periods may be played until a winner is declared.

NFL Europa , a defunct league run by the NFL, used a 10-minute overtime period, with the constraint that each team must have the opportunity of possession; once both teams have had such an opportunity, the overtime proceeds in a manner similar to the NFL's. Thus, if Team A has the first possession of overtime and scores a touchdown and converts their kick (thus being 7 points ahead of Team B), Team A would then kick off to Team B (In the NFL, the game would have ended with the touchdown, without a conversion being attempted). Team B would have to match or exceed the 7 point difference within this ensuing possession; exceeding it would end the game immediately, while matching the difference would result in a kickoff to Team A. From this point, the overtime is sudden death. The new United Football League also uses this rule.

The defunct World Football League, in its first season of 1974, used an overtime system more analogous to the system long used in international soccer. The overtime consisted of one 15-minute period, which was played in its entirety and divided into two halves of 7½ minutes each, with each half starting with a kickoff by one of the teams. The league changed to the NFL's sudden-death format for its second and last season in 1975.

In college and high school football, an overtime procedure (the Kansas plan) ensures that each team has equal opportunity to score. In college, both teams are granted possession of the ball at their opponents' 25 yard-line in succession. A coin flip takes place, with the winning team having the option either 1) to declare that they will take the ball first or second, or 2) to decide on which end of the field the series will occur (both teams' series occur on the same end of the field). The losing team will have the first option in any subsequent even-numbered overtime. In the first overtime, the team with first series attempts to score either a touchdown or a field goal; their possession ends when either a touchdown or a field goal have been scored, they turn the ball over via a fumble or an interception, or they fail to gain a first down. After a touchdown, a team may attempt either an extra-point or a two-point conversion. However, if the team on defense during the first series recovers a fumble and returns it for a touchdown, or returns an interception for a touchdown, the defensive team wins the game. (This is the only way for a college overtime game to end without both teams having possession.) Otherwise, regardless of the outcome of the first team's series (be it touchdown, field goal, or turnover), the other team begins their series. If the score remains tied after both teams have completed a series, a second overtime begins. If the score remains tied after two overtimes, teams scoring touchdowns are required to attempt a two-point conversion from the third overtime on. Just as in regulation, if a defensive team recovers a fumble/returns an interception to the end zone during a two-point conversion attempt, they will receive two points.

In high school football in a majority of states, each team is granted possession of the ball at the 10-yard line, meaning that a team cannot make a first down without scoring except via a defensive penalty that carries an automatic first down (such as defensive pass interference or roughing the passer). As is the case with the college overtime rule, the team that wins the coin toss will have the choice as to whether to take the ball first or second, or decide at which end of the field the overtime will be played. The other major difference between overtime in college football and high school football is that in high school football, if the defense forces a turnover, they may not score. However, in Texas, the college overtime rule is used, as both the University Interscholastic League, which governs interscholastic activities for Texas public high schools, and the Texas Association of Private and Parochial Schools, the largest analogous body for Texas private high schools, play by NCAA football rules with a few modifications for the high school level.

The defunct XFL used a modified Kansas Plan which, upon the first team scoring, required the opponent to score the same or greater amount of points in the same or fewer downs (i.e. if the first team scored a touchdown, and converted the one-point conversion in three downs, the opponent would have to match that touchdown and conversion in three downs as well). Each team started at the 20-yard line, but like high school, there were no opportunities for first downs. The league also banned field goals except for on a fourth down.

[edit] Playing the game

[edit] Coin toss

Three minutes before the start of the game, the referee meets with captains from both teams for a coin toss. The visiting team calls the toss. The winner of the toss may defer their choice to the start of the second half, or they may take first choice of:

1. Receiving the kickoff to start the game, or kicking off to start the game2. Choosing an end of the field to defend in the first quarter (with the teams switching directions at the end of the first

quarter and at the end of the third quarter)

The loser of the toss gets the remaining option.

At the start of the second half, the team that did not choose first (either because they deferred their choice or because they lost the toss) gets the first choice of options.

According to USA Today, in college games, the team that wins the toss defers their choice to the start of the second half over 90% of the time.[3]

If a game goes to overtime, a coin toss is held before the start of overtime, but tosses are not held before the start of subsequent overtime periods. In college, for example, the loser of the toss to start overtime has first choice in the second overtime period. The choices available to the captains in overtime vary among the NFL, college, and various states' high school rules.

In high school, the coin toss may be held between the captains or coaches earlier before the start of the game. At three minutes before kickoff, the captains meet for a simulated coin toss, where the referee announces the results of the earlier toss.

[edit] Downed player

A player carrying the ball (the runner) is downed when any of the following occurs:

Any part of the runner other than his hands or feet touches the ground. Ankles and wrists count as downed. This may be as a result of:

o Contact by an opponent (called down by contact) where the opponent tackles the runner by pushing him, grasping him and pulling him to the ground, sliding into his legs, or touching him in any manner prior to any part of the runner other than his hands or feet touching the ground. Unlike the use of the word tackle in other sports, if the opposing player fails to down the ball carrier, it is merely an attempted tackle.

o Intentionally downing the ball: intentionally kneeling, verbally declaring "I'm down" (except in college), or similar actions. For example, to protect himself from violent hits by opponents attempting to tackle him, the quarterback may choose to slide to the ground feet-first. This slide is interpreted as intentionally downing the ball, and opponents may then be penalized for hitting him.

o In amateur football, a runner is downed when any part of his body other than his hands or feet touches the ground at any time (unless he is the holder for a place kick). In professional football, the runner is not down for such accidental contact; he must be down by contact with an opponent as described above.

The runner goes out of bounds: that is, any part of his body (including his hands or feet) touches the ground, or anything other than another player or an official, on or past a sideline or an endline. Note that the sideline itself is out of bounds, so that the runner is deemed out of bounds if he steps on or touches any part of it. Note also that a runner may carry the ball in such a manner that it is over the sideline, so long as the ball or runner does not touch anything out of bounds.

The runner's forward progress toward the opponents' goal line is stopped by contact with an opponent, with little chance to be resumed. The exact moment at which the player's forward progress stops is subject to the judgment of the officials. In particular, for the protection of the quarterback, he is considered down as soon as an official judges that he is in the grasp of an opponent behind the line of scrimmage, and the tackling defensive player(s) will be awarded with a sack. If he is driven backward by the opponent, the ball will be spotted where his forward progress was stopped.

Starting the down

A scrimmage down begins with a snap, where the center throws or hands the ball backward to one of the backs, usually the quarterback. The down ends when the ball becomes dead (see below). The ball is typically next spotted where the ball became dead; however, if it became dead outside the hash marks, it is brought in on the same yard line to the nearest hash mark. This spot becomes the line of scrimmage for the next play. In the case of an incomplete forward pass, the ball is returned to the spot where it was snapped to begin the previous play. A fumbled ball that goes out of bounds is declared dead and possession remains with the team that most recently had control of the ball.

[edit] Dead ball

The ball becomes dead, and the down ends, when:

the ball carrier is downed, as described above; a forward pass falls incomplete (it touches the ground before possession is secured by a player); the ball or ball carrier touches the sideline or end line or otherwise goes outside the field of play ("out of bounds"); the ball carrier or the ball, except on a scoring field goal attempt, hits any part of the goalpost (even if it bounces

back onto the field); a team scores; a kick receiver makes a fair catch (waving his arm above his head to signal a fair catch, where the kicking team is not

allowed to interfere with him or hit him after the catch, but in return he is not allowed to run), or a member of the receiving team gains possession after a fair catch signal was given;

a member of the kicking team possesses a kicked ball beyond the line of scrimmage (e.g. "downing" a punt allowed to roll by the receiving team by holding it to stop its roll);

a kicked ball comes to rest; a touchback occurs; or under NFL or college rules, on fourth down (or, in the NFL, on any down after the two-minute warning in either half),

a ball fumbled forward by the offensive team is recovered by an offensive team player other than the fumbler.

The nearest official typically blows his whistle after the ball becomes dead to alert the players that the down has already ended. If the ball is alive and the official sounds an inadvertent whistle, then the ball still becomes dead, but the team in possession of the ball may elect to have the down replayed or take the spot where the ball was declared dead. If the ball was loose from a fumble, then the ball can be put into play at the spot of the fumble. If the ball was in flight from a kick or a pass, then the down is always replayed.

[edit] Free kick downs

A free kick is a down which does not occur from scrimmage. The kicking team begins behind the ball, while the receiving team must remain at least 10 yards downfield before the ball is kicked.

[edit] Kickoffs

A kickoff is a type of free kick where the ball is placed on a tee (or held) at the kicking team's 30-yard line (40 for high school). The kicking team's players may not cross this line until the ball is kicked; members of the non-kicking (or "receiving") team are similarly restrained behind a line 10 yards further downfield (the 40-yard line, or 50 for high school). A valid kickoff must travel at least this 10-yard distance to the receiving team's restraining line, after which any player of either team may catch or pick up the ball and try to advance it (a member of the kicking team may only recover a kickoff and may not advance it) before being downed (see "Downed player," below). In most cases, the ball is kicked as far as possible (typically 40 to 70 yards), after which a player of the receiving team is usually able to secure possession (since the members of the kicking team cannot start downfield until after the ball is kicked). Occasionally, for tactical reasons, the kicking team may instead choose to attempt an onside kick, in which the kicker tries to kick the ball along the ground just over the required 10-yard distance in such a manner that one of his own teammates can recover the ball for the kicking side.

[edit] Receiving a kickoff

A member of the receiving team gaining possession of the ball on a kickoff may attempt to advance it as far as he can toward the kicking team's goal line before being downed. Once the ball carrier is downed, the play is whistled dead and the ball is placed by the officials at the point where the play ended; this spot then becomes the line of scrimmage for the ensuing play. A kick that travels through or goes out of bounds within the end zone without being touched, or is caught by the receiving team in the end zone but not advanced out of it, results in a touchback; the ball is then placed at the receiving team's 20-yard line, which becomes the line of scrimmage.

A kickoff that goes out of bounds anywhere other than the end zone before being touched by the receiving team is an illegal kick: the receiving team has the option of having the ball re-kicked from five yards closer to the kicking team's goal line, or they may choose to take possession of the ball at the point where it went out of bounds or 30 yards from the point of the kick (25 yards in high school), whichever is more advantageous.

[edit] Other free kicks

A free kick is also used to restart the game following a safety. The team that was trapped in its own end zone, therefore conceding two points to the other team, kicks the ball from its own 20-yard line. This can be a place kick (in the NFL, a tee cannot be used), drop kick or punt.

In the NFL and high school, a free kick may be taken on the play immediately after a fair catch; see "fair catch kick" below.

[edit] Scoring

[edit] Field goal (3 points)

A field goal is scored when the ball is place kicked, drop kicked, or free kicked after a fair catch or awarded fair catch (High School or NFL only) between the goal posts behind the opponent's end zone. The most common type of kick used is the place kick. For a place kick, the ball must first be snapped to a placeholder, who holds the ball upright on the ground with his fingertip so that it may be kicked. Three points are scored if the ball crosses between the two upright posts and above the crossbar and remains over. If a field goal is missed, the ball is returned to the original line of scrimmage (in the NFL, to the spot of the kick; in high school, to the 20-yard line if the ball enters the end zone, or otherwise where the ball becomes dead after the kick) or to the 20-yard line if that is further from the goal line, and possession is given to the other team. If the ball does not go out of bounds, the other team may catch the kicked ball and attempt to advance it, but this is usually not advantageous. One official is positioned under each goalpost; if either one rules the field goal no good, then the field goal is unsuccessful. A successful field goal is signaled by an official extending both arms vertically above the head. A team that successfully kicks a field goal kicks off to the opposing team on the next play.

[edit] Touchdown (6 points)

A touchdown is achieved when a player has legal possession of the ball and the ball crosses an imaginary vertical plane above the opposing team's goal line. After a touchdown, the scoring team attempts a try for 1 or 2 points (see below). A successful touchdown is signaled by an official extending both arms vertically above the head.

For statistical purposes, the player who advances the ball into or catches it in the end zone is credited with the touchdown. If a forward pass was thrown on the play, the passer is also credited with a passing touchdown.

[edit] Try for extra point (1 or 2 points)

A try is more frequently called an extra-point attempt or a PAT (abbreviation of "Point After Touchdown"[4]). Either one or two additional points may be scored during the try. The ball is spotted at the 2 yard line (NFL) or 3 yard line (college/high school), and the team is given one untimed play to earn points:

The offensive team may attempt to kick the ball through the goalposts, in the same manner that a field goal is kicked. This option is almost always chosen, because a two point conversion attempt is much riskier. If successful the team is awarded one point, referred to as an extra point. Because the extra point is almost always successful (especially in NFL play), sportscasters will refer to a team up or trailing by seven (not six) points as being "up/trailing by a touchdown".

The offensive team may attempt to advance the ball via run or pass into the end zone, much like a touchdown on the extra-point attempt, except that it receives two points. This is called a two-point conversion. The success rate

for two-point conversions is less than 40 percent in the NFL and college, making the two-point conversion attempt a risky tactic; thus it is usually attempted only when two points will help the team but one point will not.

o For example, suppose that it is late in the game with a score of 21-10 and the losing team scores a touchdown, making the score 21-16. The scoring team will usually attempt the two-point conversion, because if successful, a three-point deficit later could be matched with one field goal; failure to convert would result in a five-point deficit that could not be surmounted without another touchdown—a situation no worse than the four-point deficit achieved with a kicked extra point.

o Another example would be if a team scores a late-game touchdown, and as a result is down by two points. A successful two-point conversion would tie the game, and likely force overtime. In very rare and risky instances, a trailing team who scored a touchdown, and as a result is down by 1 point, may attempt a two-point conversion to gamble on a win and avoid overtime.

Under college rules, if the defensive team gains possession and advances the ball the length of the field into the opposite end zone on the try (for example by an interception or a fumble recovery, or by blocking a kick and legally recovering the ball), they score two points. This is officially recorded as a touchdown scored by the defense. The NCAA adopted this rule in 1990.

o This scenario cannot occur under NFL or high school football rules; the ball is ruled dead and the try is over immediately when the defense gains possession.

In the extremely rare event that the offensive team scores a safety on the try, it receives one point. In college football, this can occur when, for example, the defense gains control of the ball and attempts to advance it, then fumbles it out of bounds in its own end zone. In the NFL, since the try ends once the defense gains possession, a safety can only be scored if a defensive player bats a loose (fumbled) ball out of bounds in the end zone.

The game clock is not advanced during a try, however, the play clock is enforced. A delay of game penalty, false start, or similar penalty, by the offense results in a 5-yard penalty assessed for the try. Typically, penalties charged against the defense give the offensive two options: half the distance to the goal for the try, or assessing the full penalty on the ensuing kickoff. Since the try is not timed by the game clock, if a touchdown is scored as regulation time expires (and game clock subsequently reads :00), the try is still allowed to be conducted. This instance is relatively common as teams score a game-tying (or game-winning) extra point after regulation has expired.

o During sudden-death overtime, particularly in the NFL, if a team scores a touchdown in the overtime period, the game is immediately over, and the try is not attempted.

The officials' signal for a successful try, whether an extra point or a two point conversion, is the same as for a touchdown. After the try, the team that scored the touchdown kicks off to the opposing team.

[edit] Safety (2 points)Main article: Safety (American football score)

The uncommon safety is scored if a player causes the ball to become dead in his own end zone; two points are awarded to the opposing (usually defending) team. This can happen if a player is either downed or goes out of bounds in the end zone while carrying the ball, or if he fumbles the ball, and it goes out of bounds in the end zone. A safety is also awarded to the defensive team if the offensive team commits a foul which is enforced in its own end zone. A safety is not awarded if a player intercepts a pass or receives a kick in his own end zone and is downed there. This situation, in which the opponent caused the ball to enter the end zone, is called a touchback; no points are scored, and the team that gained possession of the ball is awarded possession at its own 20 yard line. If the interception or reception occurs outside the end zone, and the player is carried into the end zone by momentum, the ball is placed at the spot of the catch and no safety is awarded. A safety is signaled by a referee holding both palms together above the head, fingertips pointing upwards. After a safety, the team that conceded the safety kicks a free kick (which may be a punt, place kick, or drop kick) from its 20 yard line.

[edit] Fair catch kick (High school or NFL only, 3 points)Main article: Fair catch kick

A free kick (see above) may be taken on the play immediately after any fair catch of a punt. In the NFL, if the receiving team elects to attempt this and time expired during the punt, the half is extended with an untimed down. The ball must be held on the ground by a member of the kicking team or may be drop kicked; a tee may not be used. (High school kickers may use a tee.) This is both a field goal attempt and a free kick; if the ball is kicked between the goal posts, three points are scored for the kicking team. This is the only case where a free kick may score points. This method of scoring is extremely rare; it is only advantageous when a team catches a very short punt with very little time left. Note that a team is unlikely to be punting with only a few seconds left in a half, and it is rarer still for punts to be caught near field goal range. The officials' signal for a successful fair catch kick is the same as for a field goal.

[edit] Officials

Main article: Official (American football)

The game is officiated by a crew of three to seven officials. Every crew will consist of a referee, who is generally in charge of the game and watches action on the quarterback and in the offensive backfield; an umpire, who handles spotting the ball and watches action on the offensive line; and a head linesman, who supervises placement of the down box and line-to-gain chains. The crew may also consist of a line judge, back judge, field judge and side judge, in the order listed: i.e. a crew of five officials has a referee, umpire, head linesman, line judge and back judge.

Officials are selected by the teams in advance or appointed by the governing league. Note that unlike some other sports, no football officials—not even at the NFL level—are full-time employees of the league; they all officiate on a part-time basis. In the other three major North American professional sports leagues--Major League Baseball, the NBA and NHL--officials are employed by their respective leagues. The sheer volume of games in the other three sports necessitates full-time officials; the NFL regular season is only 16 games long, compared to 162 games for MLB and 82 for the NBA and NHL.

During the game, the officials are assisted in the administration of the game by other persons, including: a clock operator to start and stop the game clock (and possibly also the play clock); a chain crew who hold the down indicator and the line-to-gain chains on the sideline; and ball boys, who provide footballs to officials between downs (e.g. a dry ball each down on a wet day). These individuals may be provided by the teams involved—it is common for a high school coach's son or daughter to act as ball boy for the team.

[edit] Fouls and their penalties

Main article: Penalty (American football)

Because football is a high-contact sport requiring a balance between offense and defense, many rules exist that regulate equality, safety, contact, and actions of players on each team. It is very difficult to always avoid violating these rules without giving up too much of an advantage. Thus, an elaborate system of fouls and penalties has been developed to "let the punishment fit the crime" and maintain a balance between following the rules and keeping a good flow of the game. Players are constantly looking for ways to find an advantage that stretches the limitations imposed by the rules. Also, the frequency and severity of fouls can make a large difference in the outcome of a game, so coaches are constantly looking for ways to minimize the number and severity of infractions committed by their players.

It is a common misconception that the term "penalty" is used to refer both to an infraction and the penal consequence of that infraction. A foul is a rule infraction for which a penalty is prescribed.[5] Some of the more common fouls are listed below. In most cases when a foul occurs, the offending team will be assessed a penalty of 5, 10 or 15 yards, depending on the foul. Also, in most cases, if the foul is committed while the ball is in play, the down will be replayed from the new position (for example, if the offense commits a foul on a first-down play, the next play will still be first down, but the offense may have to go 15 yards, or farther, to achieve another first down.) But if a defensive foul results in the ball advancing beyond the offense's first-down objective, the next play will be the first down of a new series. Some penalties (typically for more serious fouls), however, require a loss of down for the offense; and some defensive fouls may result in an automatic first down regardless of the ball position. In all cases (except for ejection of a player or, in rare cases, forfeiture of the game), the non-offending team is given the option of declining the penalty and letting the result of the play stand (although the Referee may exercise this option on their behalf when it is obvious), if they believe it to be more to their advantage. For some fouls by the defense, the penalty is applied in addition to the yardage gained on the play. Most personal fouls, which involve danger to another player, carry 15-yard penalties; in rare cases, they result in offending players being ejected from the game. In the NFL, if a defensive foul occurs after time has expired at the end of a half, the half will be continued for a single, untimed play from scrimmage. Under college rules, any accepted penalty when time has expired at the end of any quarter results in an extension for one untimed down.

In the NFL, with three exceptions, no penalty may move the ball more than half the distance toward the penalized team's goal line. These exceptions are defensive pass interference (see the discussion of that foul for more details), intentional grounding, and offensive holding – but in this last case the exception pertains only if the infraction occurs within the offensive team's own end zone, in which case an automatic safety is assessed (intentional grounding from the end zone also carries an automatic safety). Under college rules, the same half-the-distance principle applies, but any offensive fouls involving contact in their end zone (e.g. holding, illegal blocking or personal fouls) result in a safety.

Note: The neutral zone is the space between the two free-kick lines during a free-kick down and between the two scrimmage lines during a scrimmage down. For a free-kick down, the neutral zone is 10 yards wide and for a scrimmage down it is as wide as the length of the football. It is established when the ball is marked ready for play. No player may legally be in the neutral zone except for the snapper on scrimmage downs, and no one except the kicker and the holder for free kick downs.

[edit] Timeouts

Each team receives three timeouts per half (if the game goes to overtime, each team receives additional timeouts), making for a total of six timeouts per team in a regulation game. Timeouts received in the first half may not be used in the second half or overtime. In professional football, a team must have at least one remaining timeout to challenge an official's call.

Field and players

The numbers on the field indicate the number of yards to the nearest end zone.

American football is played on a field 360 by 160 feet (120.0 by 53.3 yards; 109.7 by 48.8 meters).[11] The longer boundary lines are sidelines, while the shorter boundary lines are end lines. Sidelines and end lines are out of bounds. Near each end of the field is a goal line; they are 100 yards (91.4 m) apart. A scoring area called an end zone extends 10 yards (9.1 m) beyond each goal line to each end line. The end zone includes the goal line but not the end line.[11] While the playing field is effectively flat, it is common for a field to be built with a slight crown—with the middle of the field higher than the sides—to allow water to drain from the field.

Yard lines cross the field every 5 yards (4.6 m), and are numbered every 10 yards from each goal line to the 50-yard line, or midfield (similar to a typical rugby league field). Two rows of short lines, known as inbounds lines or hash marks, run at 1-yard (91.4 cm) intervals perpendicular to the sidelines near the middle of the field. All plays start with the ball on or between the hash marks. Because of the arrangement of the lines, the field is occasionally referred to as a gridiron in a reference to the cooking grill with a similar pattern of lines.

At the back of each end zone are two goalposts (also called uprights) connected by a crossbar 10 feet (3.05 m) from the ground. For high skill levels, the posts are 18 feet 6 inches (5.64 m) apart. For lower skill levels, these are widened to 23 feet 4 inches (7.11 m).

Each team has 11 players on the field at a time. Usually there are many more players off the field. There are 53 active players on an NFL team. However, if time allows, teams may substitute for any or all of their players during the breaks between plays. As a result, players have very specialized roles and are divided into three separate units: the offense, the defense and the special teams. It is rare for all team members to participate in a given game, as some roles have little utility beyond that of an injury substitute.

Start of halves

Similarly to association football, the game begins with a coin toss to determine which team will kick off to begin the game and which goal each team will defend.[12] The options are presented again to start the second half; the choices for the first half do not automatically determine the start of the second half. The referee conducts the coin toss with the captains (or sometimes coaches) of the opposing teams. The team that wins the coin toss has three options:[12]

1. They may choose whether to kick or receive the opening kickoff.2. They may choose which goal to defend.3. They may choose to defer the first choice to the other team and have first choice to start the second half.[13]

Whatever the first team chooses, the second team has the option on the other choice (for example, if the first team elects to receive at the start of the game, the second team can decide which goal to defend).

At the start of the second half, the options to kick, receive, or choose a goal to defend are presented to the captains again. The team which did not choose first to start the first half (or which deferred its privilege to choose first) now gets first choice of options.[12][14]

Game duration

A standard football game consists of four 15-minute quarters (12-minute quarters in high-school football and often shorter at lower levels),[15] with a 12-minute half-time intermission after the second quarter.[16] At all levels, a down (play) that begins before time expires is allowed to continue until its completion, even after the clock reaches zero. The clock is also stopped after certain plays, therefore, a game can last considerably longer (often more than three

hours in real time), and if a game is broadcast on television, TV timeouts are taken at certain intervals of the game to broadcast commercials outside of game action. If an NFL game is tied after four quarters, the teams play an additional period lasting up to 15 minutes. In a regular season NFL overtime game, the first team that scores wins, even if the other team does not get a possession; this is referred to as sudden death. However, in a post-season NFL game during the playoffs, if the first team with possession scores only a field goal, the other team is allowed the opportunity to match or better this score. This rule only affects playoff games in overtime in which the first team with possession scores a field goal: if the first team with possession scores a touchdown, the sudden death rules take effect. In a regular-season NFL game, if neither team scores in overtime, the game is a tie. In an NFL playoff game, additional overtime periods are played, as needed, to determine a winner. College overtime rules are more complicated.

A line of scrimmage on the 48-yard line. The offense is on the left.

A quarterback searching for opportunity to throw a pass.

A running back being tackled when he tries to run with the ball.

A quarterback preparing to throw a pass.

Forward pass in progress, during practice.

A kicker attempts an extra point.

Advancing the ball

Advancing the ball in American football resembles the six-tackle rule and the play-the-ball in rugby league. The team that takes possession of the ball (the offense) has four attempts, called downs, in which to advance the ball at least 10 yards (9.1 m) toward their opponent's (the defense's) end zone. When the offense succeeds in gaining at least 10 yards, it gets a first down, meaning the team starts a new set of four downs to gain yet another 10 yards or to score. If the offense fails to gain a first down (10 yards) after four downs, the other team gets possession of the ball at the point where the fourth down ended, beginning with their first down to advance the ball in the opposite direction.

Except at the beginning of halves and after scores, the ball is always put into play by a snap. Offensive players line up facing defensive players at the line of scrimmage (the position on the field where the play begins). One

offensive player, the center, then passes (or "snaps") the ball backwards between his legs to a teammate behind him, usually the quarterback.

Players can then advance the ball in two ways:

1. By running with the ball, also known as rushing.2. By throwing the ball to a teammate, known as a pass or as passing the football. If the pass is thrown down-field, it is

known as a forward pass. The forward pass is a key factor distinguishing American and Canadian football from other football sports. The offense can throw the ball forward only once during a down and only from behind the line of scrimmage. However, the ball can be handed-off to another player or thrown, pitched, or tossed sideways or backwards (a lateral pass) at any time.

A down ends, and the ball becomes dead, after any of the following:

The player with the ball is forced to the ground (a tackle) or has his forward progress halted by members of the other team (as determined by an official).

A forward pass flies beyond the dimensions of the field (out of bounds) or touches the ground before it is caught. This is known as an incomplete pass. The ball is returned to the most recent line of scrimmage for the next down.

The ball or the player with the ball goes out of bounds. A team scores.

Officials blow a whistle to notify players that the down is over.

Before each down, each team chooses a play, or coordinated movements and actions, that the players should follow on a down. Sometimes, downs themselves are referred to as "plays."

Change of possession

The offense maintains possession of the ball unless one of the following things occurs:

The team fails to get a first down— i.e., in four downs they fail to move the ball past a line 10 yards ahead of where they got their last first down. The defensive team takes over the ball at the spot where the 4th-down play ends. A change of possession in this manner is commonly called a turnover on downs.

The offense scores a touchdown or field goal. The team that scored then kicks the ball to the other team in a special play called a kickoff.

The offense punts the ball to the defense. A punt is a kick in which a player drops the ball and kicks it before it hits the ground. Punts are nearly always made on fourth down, when the offensive team does not want to risk giving up the ball to the other team at its current spot on the field (through a failed attempt to make a first down) and feels it is too far from the other team's goal post to attempt a field goal.

A defensive player catches a forward pass. This is called an interception, and the player who makes the interception can run with the ball until he is tackled, forced out of bounds, or scores.

An offensive player drops the ball (a fumble) and a defensive player picks it up. As with interceptions, a player recovering a fumble can run with the ball until tackled, forced out of bounds, or scoring. Passes that are thrown either backwards or parallel with the line of scrimmage (lateral passes) that are not caught do not cause the down to end as incomplete forward passes do; instead the ball is still live as if it had been fumbled. Lost fumbles and interceptions are together known as turnovers.

The offensive team misses a field goal attempt. The defensive team gets the ball at the spot where the previous play began (or, in the NFL, at the spot of the kick). If the unsuccessful kick was attempted from within 20 yards (18.3 m) of the end zone, the other team gets the ball at its own 20 yard line (that is, 20 yards from the end zone). If a field goal is missed or blocked and the ball remains in the field of play, a defensive player may pick up the ball and attempt to advance it.

While in his own end zone, an offensive ball carrier is tackled, forced out of bounds, loses the ball out of bounds, or the offense commits certain fouls in the end zone. This fairly rare occurrence is called a safety.

An offensive ball carrier fumbles the ball forward into the opposing end zone, and then the ball goes out of bounds. This extremely rare occurrence leads to a touchback, with the ball going over to the opposing team at their 20 yard line (Note that touchbacks during non-offensive special teams plays, such as punts and kickoffs, are quite common).

ScoringMain article: American_football_rules#Scoring

A team scores points by the following plays:

A touchdown (TD) is worth 6 points.[16] It is scored when a player runs the ball into or catches a pass in his opponent's end zone.[16] A touchdown is analogous to a try in rugby. Unlike rugby, a player does not have to touch the ball to the ground to score; a touchdown is scored any time a player has possession of the ball while any part of the ball is beyond the vertical plane created by the leading edge of the opponent's goal line stripe (the stripe itself is a part of the end zone).

o After a touchdown, the scoring team attempts a try (which is also analogous to the conversion in rugby). The ball is placed at the other team's 3-yard (2.7 m) line (the 2-yard (1.8 m) line in the NFL). The team can attempt to kick it through the goalposts (over the crossbar and between the uprights) in the manner of a field goal for 1 point (an extra point or point-after touchdown (PAT)[17]), or run or pass it into the end zone in the manner of a touchdown for 2 points (a two-point conversion). In college football, if the defense intercepts or recovers a fumble during a one or two point conversion attempt and returns it to the opposing end zone, the defensive team is awarded the two points.

A field goal (FG) is worth 3 points, and it is scored by kicking the ball through the goalposts defended by the opposition.[16] Field goals may be place kicked (kicked when the ball is held vertically against the ground by a teammate) or drop kicked (extremely uncommon in the modern game due to the better accuracy of place kicks, with only two successful drop kicks in sixty-plus years in the NFL). A field goal is usually attempted on fourth down instead of a punt when the ball is close enough to the opponent's goalposts, or, when there is little or no time left to otherwise score.

A safety, worth 2 points, is scored by the opposing team when the team in possession at the end of a down is responsible for the ball becoming dead behind its own goal line. For instance, a safety is scored by the defense if an offensive player is tackled, goes out of bounds, or fumbles the ball out of bounds in his own end zone.[16] Safeties are relatively rare. Note that, though even more rare, the team initially on offense during a down can score a safety if a player of the original defense gains possession of the ball in front of his own goal line and then carries the ball or fumbles it into his own end zone where it becomes dead. However, if the ball becomes dead behind the goal line of the team in possession and its opponent is responsible for the ball being there (for instance, if the defense intercepts a forward pass in its own end zone and the ball becomes dead before the ball is advanced out of the end zone) it is a touchback: no points are scored and the team last in possession keeps possession with a first down at its own 20 yard line. In amateur football, in the extremely rare instance that a safety is scored on a try, it is worth only 1 point.

Kickoffs and free kicks

The Florida State Seminoles (in red, at top) line up to kick off to the Virginia Tech Hokies.

Each half begins with a kickoff. Teams also kick off after scoring touchdowns and field goals. The ball is kicked using a kicking tee from the team's own 30-yard (27 m) line in the NFL and college football (as of the 2007 season). The other team's kick returner tries to catch the ball and advance it as far as possible. Where he is stopped is the point where the offense will begin its drive, or series of offensive plays. If the kick returner catches the ball in his own end zone, he can either run with the ball, or elect for a touchback by kneeling in the end zone, in which case the receiving team then starts its offensive drive from its own 20 yard line. A touchback also occurs when the kick goes out-of-bounds in the end zone. (Punts and turnovers in the end zone can also result in a touchback). A kickoff that goes out-of-bounds anywhere other than the end zone before being touched by the receiving team is a foul, and the ball will be placed within the hash marks of the yard line where it went out of bounds, or 30 yards (27 m) from the kickoff spot, depending on which is more advantageous to the receiving team.[18] Unlike with punts, once a kickoff goes 10 yards and the ball has hit the ground, it can be recovered by the kicking team.[18] A team, especially one who is losing, can try to take advantage of this by attempting an onside kick.

After safeties, the team that gave up the points must free kick the ball to the other team from its own 20 yard line.[19]

Penalties

A penalty flag on the field during a game on November 16, 2008 between the San Francisco 49ers and St. Louis Rams.

Main article: Penalty (American football)

Fouls (a type of rule violation) are punished with penalties against the offending team. Most penalties result in moving the football towards the offending team's end zone. If the penalty would move the ball more than half the distance towards the offender's end zone, the penalty becomes half the distance to the goal instead of its normal value.

Most penalties result in replaying the down. Some defensive penalties give the offense an automatic first down.[20] Conversely, some offensive penalties result in loss of a down (loss of the right to repeat the down).[20] If a penalty gives the offensive team enough yardage to gain a first down, they get a first down, as usual. The only penalty that results in points is if a team on offense commits a certain fouls, such as holding, in its own end zone, which results in a safety.

If a foul occurs during a down (after the play has begun), the down is allowed to continue and an official throws a yellow penalty flag near the spot of the foul. When the down ends, the team that did not commit the foul has the option of accepting the penalty, or declining the penalty and accepting the result of the down.

VariationsSee also: List of gridiron football codes

Limited contact o touch football . A play ends when a defender touches the ball carrier (sometimes with two hands).o flag football . A play ends when a defender removes a designated token ("flag") worn by the ball carrier.o Wrap. A play ends when a defender wraps his arms round the ball carrier.

Fewer players o nine-man football o eight-man football o six-man football

Smaller field

The Arena Football League is a league that plays eight-man football, but also plays indoors and on a much smaller playing surface with rule changes to encourage a much more offensive game.

Catch and Run

In this game, the children split into two teams and line up at opposite sides of the playing field. One side throws the ball to the other side. If the opposing team catches the ball, that player tries to run to the throwing teams touchdown without being tagged/tackled. If no one catches the ball or if the player is tagged/tackled, then that team has to throw the ball to the opposing team. This repeats until time runs out or the players decide to quit.

Players

This diagram shows typical offensive and defensive formations. Because teams can change any or all of the players between plays, the number of players at certain positions may differ on a given play. Here the offense is in the Normal I-Formation while the defense is in a 4-3 Normal.

Main article: American football positions

See also: Formation (American football)

Most football players have highly specialized roles. At the college and NFL levels, most play only offense or only defense.

Offense

The offensive line (OL) consists of five players whose job is to protect the passer and clear the way for runners by blocking members of the defense. The lineman in the middle is the Center. Outside the Center are the Guards, and outside them are the Tackles. Except for the center, who snaps the ball to one of the backs, offensive linemen generally do not handle the ball.

The quarterback (QB) receives the snap from the center on most plays. He then hands or tosses it to a running back, throws it to a receiver or runs with it himself. The quarterback is the leader of the offense and calls the plays that are signaled to him from the sidelines.

Running backs (RB) line up behind or beside the QB and specialize in running with the ball. They also block, catch passes and, on rare occasions, pass the ball to others or even receive the snap. If a team has two running backs in the game, usually one will be a halfback (HB) (or tailback (TB)), who is more likely to run with the ball, and the other will usually be a fullback (FB), who is more likely to block.

Wide receivers (WR) line up near the sidelines. They specialize in catching passes, though they also block for running plays or downfield after another receiver makes a catch.

Tight ends (TE) line up outside the offensive line. They can either play like wide receivers (catch passes) or like offensive linemen (protect the QB or create spaces for runners). Sometimes an offensive lineman takes the tight end position and is referred to as a tackle eligible.[21]

At least seven players must line up on the line of scrimmage on every offensive play. The other players may line up anywhere behind the line. The exact number of running backs, wide receivers and tight ends may differ on any given play. For example, if the team needs only one yard, it may use three tight ends, two running backs and no wide receivers. On the other hand, if it needs 20 yards, it may replace all of its running backs and tight ends with wide receivers.

Defense

In contrast to members of the offense, the rules of professional football (NFL Rulebook) and American college football (NCAA Rulebook) do not specify starting position, movement, or coverage zones for members of the defensive team, except that they must be in the defensive zone at the start of play. The positions, movements and responsibilities of all defensive players are assigned by the team by selection of certain coverages, or patterns of placement and assignment of responsibilities. The positional roles are customary. These roles have varied over the history of American football. The following are customary defensive positions used in many coverages in modern American football.

The defensive line consists of three to six players who line up immediately across from the offensive line. They try to occupy the offensive linemen in order to free up the linebackers, disrupt the backfield (behind the offensive line) of the offense, and tackle the running back if he has the ball before he can gain yardage or the quarterback before he can throw or pass the ball. They are the first line of defense.

Behind the defensive line are the linebackers. They line up between the defensive line and defensive backs and may either rush the quarterback or cover potential receivers.

The last line of defense is known as the secondary, comprising at least three players who line up as defensive backs, who are either cornerbacks or safeties. They cover the receivers and try to stop pass completions. They occasionally rush the quarterback.

Special teams

The units of players who handle kicking plays are known as special teams. Three important special-teams players are the punter, who handles punts, the placekicker or kicker, who kicks off and attempts field goals and extra points, and the long snapper, who snaps the ball for extra points, field goals, and punts. Also included on special teams are the returners. These players return punts or kickoffs and try to get in good field position. These players can also score touchdowns.

Uniform numbering

Quarterback Shaun Carney has uniform number 5.

In the NFL, ranges of uniform numbers are (usually) reserved for certain positions:

1–19: Quarterbacks, punters and placekickers[22]

20–49: Running backs and defensive backs[23]

50–79: Offensive and defensive linemen[24]

10–19, 80–89: Wide receivers[25]

40–49, 80–89: Tight ends[26]

50–59, 90–99: Linebackers and defensive linemen (90–99)[27]

Players who switch positions in their career can keep their number if they played their prior position for at least a year and move from a position that is eligible to receive passes to another eligible position, or if he is moving from one ineligible position to another ineligible position.[28]

NCAA and high school rules specify only that offensive linemen must have numbers in the 50–79 range, but the NCAA "strongly recommends" that quarterbacks and running backs have numbers below 50 and wide receivers numbers above 79.[29] This helps officials, as it means that numbers 50 to 79 are ineligible receivers, or players that may not receive a forward pass (except in the rare instance when a Tackle lines up as the outermost lineman on his side of the line and the officials are notified that he will be an eligible receiver for that particular play). There are no numbering restrictions on defensive players in the NCAA, other than that a team may not have two players on the field at the same time with the same jersey number.[30]