American Football

American football, referred to as football in the United States and Canada and also known as gridiron, is a team sport played by two teams of eleven players on a rectangular field with goalposts at each end. The offense, which is the team controlling the oval-shaped football, attempts to advance down the field by running with or passing the ball, while the defense, which is the team without control of the ball, aims to stop the offense’s advance and aims to take control of the ball for themselves. The offense must advance at least ten yards in four downs, or plays, and otherwise they turn over the football to the defense; if the offense succeeds in advancing ten yards or more, they are given a new set of four downs. Points are primarily scored by advancing the ball into the opposing team’s end zone for a touchdown or kicking the ball through the opponent’s goalposts for a field goal. The team with the most points at the end of a game wins.

American football evolved in the United States, originating from the sports of association football (known in the U.S. as soccer) and rugby football. The first match of American football was played on November 6, 1869, between two college teams, Rutgers and Princeton, under rules based on the association football rules of the time. During the latter half of the 1870s, colleges playing association football switched to the Rugby Union code, which allowed carrying the ball. A set of rule changes drawn up from 1880 onward by Walter Camp, the “Father of American Football”, established the snap, eleven-player teams, and the concept of downs; later rule changes legalized the forward pass, created the neutral zone, and specified the size and shape of the football. The sport is closely related to Canadian football, which evolved parallel and contemporary to the American game, and most of the features that distinguish American football from rugby and soccer are also present in Canadian football.

A football game is played between two teams of 11 players each.[31][32][33] Playing with more on the field is punishable by a penalty. Teams may substitute any number of their players between downs; this “platoon” system replaced the original system, which featured limited substitution rules, and has resulted in teams utilizing specialized offensive, defensive and special teams squads.

Individual players in a football game must be designated with a uniform number between 1 and 99. NFL teams are required to number their players by a league-approved numbering system, and any exceptions must be approved by the Commissioner. NCAA and NFHS teams are “strongly advised” to number their offensive players according to a league-suggested numbering scheme.

Offensive unit
The role of the offensive unit is to advance the football down the field with the ultimate goal of scoring a touchdown.

The offensive team must line up in a legal formation before they can snap the ball. An offensive formation is considered illegal if there are more than four players in the backfield or fewer than five players numbered 50–79 on the offensive line. Players can temporarily line up in a position whose eligibility is different from what their number permits as long as they immediately report the change to the referee, who then informs the defensive team of the change. Neither team’s players, with the exception of the snapper, are allowed to line up in or cross the neutral zone until the ball is snapped. Interior offensive linemen are not allowed to move until the snap of the ball.
The main backfield positions are the quarterback (QB), halfback/tailback (HB/TB) and fullback (FB). The quarterback is the leader of the offense. Either the quarterback or a coach calls the plays. Quarterbacks typically inform the rest of the offense of the play in the huddle before the team lines up. The quarterback lines up behind the center to take the snap and then hands the ball off, throws it or runs with it.

The primary role of the halfback, also known as the running back or tailback, is to carry the ball on running plays. Halfbacks may also serve as receivers. Fullbacks tend to be larger than halfbacks and function primarily as blockers, but they are sometimes used as runners in short-yardage situations and are seldom used in passing situations.

The offensive line (OL) consists of several players whose primary function is to block members of the defensive line from tackling the ball carrier on running plays or sacking the quarterback on passing plays. The leader of the offensive line is the center (C), who is responsible for snapping the ball to the quarterback, blocking, and for making sure that the other linemen do their jobs during the play. On either side of the center are the guards (G), while tackles (T) line up outside the guards.

The principal receivers are the wide receivers (WR) and the tight ends (TE).[50] Wide receivers line up on or near the line of scrimmage, split outside the line. The main goal of the wide receiver is to catch passes thrown by the quarterback, but they may also function as decoys or as blockers during running plays. Tight ends line up outside the tackles and function both as receivers and as blockers.

Defensive unit

The role of the defense is to prevent the offense from scoring by tackling the ball carrier or by forcing turnovers (interceptions or fumbles).
The defensive line (DL) consists of defensive ends (DE) and defensive tackles (DT). Defensive ends line up on the ends of the line, while defensive tackles line up inside, between the defensive ends. The primary responsibilities of defensive ends and defensive tackles is to stop running plays on the outside and inside, respectively, to pressure the quarterback on passing plays, and to occupy the line so that the linebackers can break through.

Line backers line up behind the defensive line but in front of the defensive backfield. They are divided into two types: middle linebackers (MLB) and outside linebackers (OLB). Linebackers are the defensive leaders and call the defensive plays. Their diverse roles include defending the run, pressuring the quarterback, and guarding backs, wide receivers and tight ends in the passing game.

The defensive backfield, often called the secondary, consists of cornerbacks (CB) and safeties (S). Safeties are themselves divided into free safeties (FS) and strong safeties (SS). Cornerbacks line up outside the defensive formation, typically opposite of a receiver so as to be able to cover him, while safeties line up between the cornerbacks but farther back in the secondary. Safeties are the last line of defense, and are responsible for stopping deep passing plays as well as running plays.

Special teams unit

The special teams unit is responsible for all kicking plays. The special teams unit of the team in control of the ball will try and execute field goal (FG) attempts, punts and kickoffs, while the opposing team’s unit will aim to block or return them.
Three positions are specific to the field goal and PAT (point-after-touchdown) unit: the placekicker (K or PK), holder (H) and long snapper (LS). The long snapper’s job is to snap the football to the holder, who will catch and position it for the placekicker. There is not usually a holder on kickoffs, because the ball is kicked off of a tee; however, a holder may be used in certain situations, such as if wind is preventing the ball from remaining upright on the tee. The player on the receiving team who catches the ball is known as the kickoff returner (KR).

The positions specific to punt plays are the punter (P), long snapper, upback and gunner. The long snapper snaps the football directly to the punter, who then drops and kicks it before it hits the ground. Gunners line up split outside the line and race down the field, aiming to tackle the punt returner (PR) – the player that catches the punt. Upbacks line up a short distance behind the line of scrimmage, providing additional protection to the punter.

RULES

Scoring
In American football, the winner is the team that has scored the most points at the end of the game. There are multiple ways to score in a football game.
The touchdown (TD), worth six points, is the most valuable scoring play in American football. A touchdown is scored when a live ball is advanced into, caught in, or recovered in the end zone of the opposing team. The scoring team then attempts a try or conversion, more commonly known as the point(s)-after-touchdown (PAT), which is a single scoring opportunity.
A PAT is most commonly attempted from the two- or three-yard line, depending on the level of play. If scored by a placekick or dropkick through the goal posts, it is worth one point, and is typically called the extra point. Conversely, a team may elect to run a single play from scrimmage and attempt, once again, to achieve what would normally be considered a touchdown. In such a case, a successful attempt is called the two-point conversion and is worth two points. For the 2015 season, the NFL adopted a rules on PATs that stated during an extra point the placekick must be snapped from the 15-yard line and on extra points if the kick is blocked and the opposing team returns it into the end zone or if during a two-point conversion the ball is fumbled or intercepted and returned to the end zone the opposing team will score two points. No points are awarded on a failed extra point or two-point conversion attempt, although under a rare set of circumstances it is possible to score a safety, worth one point, if the defense takes the ball back into its own end zone and is downed there. In general, the extra point is almost always successful in professional play and is only slightly less successful at amateur levels, while the two-point conversion is a much riskier play with a higher probability of failure; accordingly, extra point attempts are far more common than two-point conversion attempts.
A field goal (FG), worth three points, is scored when the ball is placekicked or dropkicked through the uprights and over the crossbars of the defense’s goalposts. After a PAT attempt or successful field goal the scoring team must kick the ball off to the other team.
A safety is scored when the ball carrier is tackled in their own end zone. Safeties are worth two points, which are awarded to the defense. In addition, the team that conceded the safety must kick the ball to the scoring team via a free kick.
Field and equipment
Football games are played on a rectangular field that measures 120 yards (110 m) long and 53.33 yards (48.76 m) wide. Lines marked along the ends and sides of the field are known respectively as the end lines and sidelines, and goal lines are marked 10 yards (9.1 m) inward from each end line. Weighted pylons are placed on the inside corner of the intersections of the goal lines and end lines.
White markings on the field identify the distance from the end zone. Inbound lines, or hash marks, are short parallel lines that mark off 1 yard (0.91 m) increments. Yard lines, which can run the width of the field, are marked every 5 yards (4.6 m). A one-yard-wide line is placed at each end of the field; this line is marked at the center of the two-yard line in professional play and at the three-yard line in college play. Numerals that display the distance from the closest goal line in yards are placed on both sides of the field every ten yards.
Goalposts are located at the center of the plane of each of the two end lines. The crossbar of these posts is ten feet (3.0 meters) above the ground, with vertical uprights at the end of the crossbar 18 feet 6 inches (5.64 m) apart for professional and collegiate play and 23 feet 4 inches (7.11 m) apart for high school play. The uprights extend vertically 35 feet on professional fields, a minimum of 10 yards on college fields, and a minimum of ten feet on high school fields. Goal posts are padded at the base, and orange ribbons are normally placed at the tip of each upright.
The football itself is an oval ball, similar to the balls used in rugby or Australian rules football.[69] At all levels of play, the football is inflated to 12 1⁄2 to 13 1⁄2 pounds per square inch (psi) and weighs 14 to 15 ounces (397 to 425 grams); beyond that, the exact dimensions vary slightly. In professional play the ball has a long axis of 11 to 11 1⁄4 inches, a long circumference of 28 to 28 1⁄2 inches, and a short circumference of 21 to 21 1⁄4 inches, while in college and high school play the ball has a long axis of 10 7⁄8 to 11 7⁄16 inches, a long circumference of 27 3⁄4 to 28 1⁄2 inches, and a short circumference of 20 3⁄4 to 21 1⁄4 inches.
Duration and time stoppages
Football games last for a total of 60 minutes in professional and college play and are divided into two-halves of 30 minutes and four-quarters of 15 minutes. High school football games are 48 minutes in length with two-halves of 24 minutes and four-quarters of 12 minutes. The two-halves are separated by a halftime period, and the first and third quarters are also followed by a short break. Prior to the start of the game, the referee and team captains for each team meet at midfield for a coin toss. The visiting team is allowed to call ‘heads’ or ‘tails’; the winner of the toss is allowed to decide between choosing whether to receive or kick off the ball or choosing which goal they want to defend, but they can also defer their choice until the second half. The losing team, unless the winning team decides to defer, is allowed to choose the option the winning team did not select, and receives the option to receive, kick, or select a goal to defend to begin the second half. Most teams choose to receive or defer, because choosing to kick the ball to start the game would allow the other team to choose which goal to defend.[77] Teams switch goals following the first and third quarters. If a down is in progress when a quarter ends, play continues until the down is completed.
Games last longer than their defined length due to play stoppages – the average NFL game lasts slightly over three hours. Time in a football game is measured by the game clock. An operator is responsible for starting, stopping and operating the game clock based on the direction of the appropriate official. A separate clock, the play clock, is used to determine if a delay of game infraction has been committed. If the play clock expires before the ball has been snapped or free-kicked, a delay of game foul is called on the offense. The play clock is set to 40 seconds in professional and college football and to 25 seconds in high school play or following certain administrative stoppages in the former levels of play.

Advancing the ball and downs

There are two main ways that the offense can advance the ball: running and passing. In a typical play, the quarterback calls the play, and the center passes the ball backwards and under their legs to the quarterback in a process known as the snap. The quarterback then either hands the ball off to a back, throws the ball or runs with it. The play ends when the player with the ball is tackled or goes out of bounds, or a pass hits the ground without a player having caught it. A forward pass can only be legally attempted if the passer is behind the line of scrimmage. In the NFL, a down also ends if the runner’s helmet comes off.
The offense is given a series of four plays, known as downs. If the offense advances ten or more yards in the four downs, they are awarded a new set of four downs. If they fail to advance ten yards, possession of the football is turned over to the defense. In most situations, if the offense reaches their fourth down they will punt the ball to the other team, which forces them to begin their drive from further down the field; if they are in field goal range, they might also attempt to score a field goal. A group of officials, the chain crew, keeps track of both the downs and the distance measurements. On television, a yellow line is electronically superimposed on the field to show the first down line to the viewing audience.
Kicking
There are two categories of kicks in football: scrimmage kicks, which can be executed by the offensive team on any down from behind or on the line of scrimmage, and free kicks. The free kicks are the kickoff, which starts the first and third quarters and overtime and follows a try attempt or a successful field goal, and the safety kick, which follows a safety.
On a kickoff, the ball is placed at the 35-yard line of the kicking team in professional and college play and at the 40-yard line in high school play. The ball may be drop-kicked or place-kicked. If a place kick is chosen, the ball can be placed on the ground or on a tee, and a holder may be used in either case. On a safety kick, the kicking team kicks the ball from their own 20-yard line. They can punt, drop-kick or place-kick the ball, but a tee may not be used in professional play. Any member of the receiving team may catch or advance the ball, and the ball may be recovered by the kicking team once it has gone at least ten yards and has touched the ground or has been touched by any member of the receiving team.
The three types of scrimmage kicks are place kicks, drop kicks, and punts. Only place kicks and drop kicks can score points. The place kick is the standard method used to score points, because the pointy shape of the football makes it difficult to reliably drop kick. Once the ball has been kicked from a scrimmage kick, it can be advanced by the kicking team only if it is caught or recovered behind the line of scrimmage. If it is touched or recovered by the kicking team beyond this line, it becomes dead at the spot where it was touched. The kicking team is prohibited from interfering with the receiver’s opportunity to catch the ball, and the receiving team has the option of signaling for a fair catch. This prohibits the defense from blocking into or tackling the receiver, but the play ends as soon as the ball is caught and the ball may not be advanced.
Officials and fouls
Officials are responsible for enforcing game rules and monitoring the clock. All officials carry a whistle and wear black-and-white striped shirts and black hats except for the referee, whose hat is white. Each carries a weighted yellow flag that is thrown to the ground to signal that a foul has been called. An official who spots multiple fouls will throw their hat as a secondary signal. The seven officials on the field are each tasked with a different set of responsibilities:
• The referee is positioned behind and to the side of the offensive backs. The referee is charged with oversight and control of the game and is the authority on the score, the down number, and any and all rule interpretations in discussions between the other officials. The referee announces all penalties and discusses the infraction with the offending team’s captain, monitors for illegal hits against the quarterback, makes requests for first-down measurements, and notifies the head coach whenever a player is ejected. The referee positions themselves to the passing arm side of the quarterback. In most games, the referee is responsible for spotting the football prior to a play from scrimmage.
• The umpire is positioned in the defensive backfield, except in the NFL, where the umpire is positioned lateral to the referee on the opposite side of the formation. The umpire watches play along the line of scrimmage to make sure that no more than 11 offensive players are on the field prior to the snap and that no offensive linemen are illegally downfield on pass plays. The umpire monitors the contact between offensive and defensive linemen and calls most of the holding penalties. The umpire records the number of timeouts taken and the winner of the coin toss and the game score, assists the referee in situations involving possession of the ball close to the line of scrimmage, determines whether player equipment is legal, and dries wet balls prior to the snap if a game is played in rain.
• The back judge is positioned deep in the defensive backfield, behind the umpire. The back judge ensures that the defensive team has no more than 11 players on the field and determines whether catches are legal, whether field goal or extra point attempts are good, and whether a pass interference violation occurred. The back judge is also responsible for the play clock, the time between each play, when a visible play clock is not used.
• The head linesman is positioned on one end of the line of scrimmage. The head linesman watches for any line-of-scrimmage and illegal use-of-hands violations and assists the line judge with illegal shift or illegal motion calls. The head linesman also rules on out-of-bounds calls that happen on their side of the field, oversees the chain crew and marks the forward progress of a runner when a play has been whistled dead.
• The side judge is positioned twenty yards downfield of the head linesman. The side judge mainly duplicates the functions of the field judge. On field goal and extra point attempts, the side judge is positioned lateral to the umpire.
• The line judge is positioned on the end of the line of scrimmage, opposite the head linesman. He or she supervises player substitutions, the line of scrimmage during punts, and game timing. The line judge notifies the referee when time has expired at the end of a quarter and notifies the head coach of the home team when five minutes remain for halftime. In the NFL, the line judge also alerts the referee when two minutes remain in the half. If the clock malfunctions or becomes inoperable, the line judge becomes the official timekeeper.
• The field judge is positioned twenty yards downfield from the line judge. The field judge monitors and controls the play clock, counts the number of defensive players on the field, and watches for offensive pass interference and illegal use-of-hands violations by offensive players. The field judge also makes decisions regarding catches, recoveries, the ball spot when a player goes out of bounds, and illegal touching of fumbled balls that have crossed the line of scrimmage. On field goal and extra point attempts, the field judge is stationed under the upright opposite the back judge.
• The center judge is an eighth official used in the top level of college football. The center judge stands lateral to the referee, the same way the umpire does in the NFL. The center judge is responsible for spotting the football after each play, and has many of the same responsibilities as the referee, except announcing penalties.
Another set of officials, the chain crew, are responsible for moving the chains. The chains, consisting of two large sticks with a 10-yard-long chain between them, are used to measure for a first down. The chain crew stays on the sidelines during the game, but if requested by the officials they will briefly bring the chains on to the field to measure. A typical chain crew will have at least three people – two members of the chain crew will hold either of the two sticks, while a third will hold the down marker. The down marker, a large stick with a dial on it, is flipped after each play to indicate the current down, and is typically moved to the approximate spot of the ball. The chain crew system has been used for over 100 years and is considered to be an accurate measure of distance, rarely subject to criticism from either side.

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