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RULES

Field and players | Game duration | Advancing the ball | Changes of possession| Scoring | Kickoffs and free kicks | Penalties | Some common penalties

The object of American football is to score more points than the opposing team within a set time limit.

Field and players

The numbers on the field indicate the number of yards to the nearest end zone.The field is often called the gridiron because the markings on the field resemble a grill. The game is played on a rectangular field 120 yards (110 metres) long by 53 1/3 yards (49 metres) wide. The longer boundary lines are sidelines, while the shorter boundary lines are end lines. Near each end of the field is a goal line; they are 100 yards apart. A scoring area called an end zone extends 10 yards beyond each goal line to each end line.
Yard lines cross the field every 5 yards, and are numbered from each goal line to the 50-yard line, or midfield (similar to a typical rugby league field). Two rows of lines, known as hash marks parallel the side lines near the middle of the field. All plays start with the ball on or between the hash marks.
At the back of each end zone are two goal posts (also called uprights) that are 18.5 feet apart. The posts are connected by a crossbar 10 feet from the ground. Successful kicks must go above the crossbar and between the uprights. (At many fields the uprights and crossbar are attached by a curved bar to a post outside the field of play, to reduce the chance of players running into the supports).
Each team has 11 players on the field at a time. However, teams may substitute for any or all of their players between plays. As a result, players have very specialized roles, and almost all of the 53 players on an NFL team will play in any given game. Thus, teams are divided into three separate units: the offense, the defense and the special teams (see below). In the NFL, players' jersey numbers are distributed according to a strict system (e.g. quarterbacks always wear between 1-19).

Game duration

A standard football game consists of four 15-minute (typically 12 minutes in high school football) periods (called quarters), with an intermission (called halftime) after the second quarter. The clock stops after certain plays; therefore, a game can last considerably longer (often more than three hours in real time). If an NFL game is tied after four quarters, the teams play up to another 15 minutes. In an NFL overtime game, the first team that scores wins; if neither team scores, the game is a tie. College overtime rules are more complicated and are described at Overtime (sport).

Advancing the ball

Advancing the ball in American football resembles the six-tackle rule and the play-the-ball in rugby league football. The team that takes possession of the ball (the offense) has four attempts, called downs, to advance the ball 10 yards towards their opponent's (the defense's) end zone. When the offense gains 10 yards, it gets a first down, or another set of four downs to gain 10 yards. If the offense fails to gain a first down (10 yards) after 4 downs, it loses possession of the ball.
Except at the beginning of halves and after scores (see Kickoffs and free kicks below), the ball is always put into play by a snap. All players line up facing each other 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 between his legs to a teammate, usually the quarterback.
Players can then advance the ball in two ways:
* By running with the ball, also known as rushing.
* By throwing the ball to a teammate, known as passing. 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 on a play and only from behind the line of scrimmage. The ball can be thrown sideways or backwards at any time. This type of pass is known as a lateral and is much rarer in American football than in rugby league or rugby union, where a backwards pass is mandatory.
A play or down ends, and the ball becomes dead, after any of the following:
The player with the ball is forced to the ground or has his forward progress halted by members of the other team (as determined by an official).
A forward pass flies out of bounds or touches the ground before it is caught. This is known as an incomplete pass. The ball is returned to the original line of scrimmage for the next down.
The ball or the player with the ball goes beyond the dimensions of the field (out of bounds).A team scores.
A certain penalty is committed (such as false start) that causes the play to be blown dead and replayed. Officials blow a whistle to notify all players that the play is over.
At all times, players and fans must be aware of the sequence of downs and the distance to a new first down. When a team has a first down, the scoreboard or television screen flashes "1st and 10" — that is, first down and 10 yards to go. If the team gains three yards on the first play, for example, the next down will be "2nd and 7."

Changes of possession

The offense maintains possession of the ball unless one of the following things happens:
The team fails to get a first down, that is, move the ball forward at least 10 yards in four downs. The defensive team takes over the ball at the spot where the 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 off the ball to the other team. (See Scoring and Kickoffs below).
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 posts to kick 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 tackled, forced out of bounds, or scores. After the intercepting player is tackled or forced out of bounds, his team's offensive unit returns to the field and takes over at his last position. 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 or forced out of bounds. 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 of the end zone, the other team gets the ball at its own 20-yard line (that is, 20 yards from the end zone). An offensive player is tackled, forced out of bounds, or commits certain penalties in his own end zone. This rare occurrence is called a safety. (See Scoring below).

Scoring

A team scores points by the following plays:
A touchdown (TD) is worth 6 points. A touchdown is scored when a player runs the ball into or catches a pass in his opponent's end zone. After a touchdown, the scoring team attempts a conversion. The ball is placed at the other team's 3-yard-line (the 2-yard-line in the NFL). The team can attempt to kick it over the crossbar and through the goal posts in the manner of a field goal for 1 point (an extra point), or run or pass it into the end zone in the manner of a touchdown for 2 points (a two-point conversion). In collegiate and professional leagues, the extra point is usually preferred; its success rate is 94% in the NFL and 93.8% in the NCAA, compared to 43% in the NFL and 43.5% in the NCAA for two-point conversions. If the defense forces a turnover on an attempted conversion and runs the ball back to their opponent's endzone, they are awarded with 2 points (does not apply in the NFL).
A field goal (FG) is worth 3 points, and it is scored by kicking the ball over the crossbar and through the goal posts. Field goals may be placekicked (kicked when the ball is held vertically against the ground by a teammate) or drop-kicked. A field goal is usually attempted on fourth down instead of a punt when the ball is close to the goal line, or, when there is little or no time left to otherwise score.
A safety is worth 2 points. A safety is scored by the defense when the offensive player in possession of the ball is forced back into his own end zone and is tackled there, or fumbles the ball out of the end zone. Certain penalties by the offense occurring in the end zone also result in a safety.

Kickoffs and free kicks

Each half begins with a kickoff. Teams also kick off after scoring touchdowns and field goals. The ball is kicked from a kicking tee, which is made from one's own 30-yard line in the NFL and from the 35-yard line in college football. 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 a kick returner does not want to run with the ball, he has the option to signal for a "fair catch" by waving his hands in the air before the catch. He will then be allowed to catch the ball and kneel it down on the field without being tackled. 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. The receiving team can then start its offensive drive from its own 20-yard line. A touchback can also occur when the kick goes out of the end zone. Punts and turnovers in the end zone can also end in touchbacks. If a kickoff goes out of bounds over the sidelines without being interfered by the receiving team, the ball will be placed 30 yards from the spot of the kickoff (traditionally at the receiving team's 40-yard line in the NFL or the 35-yard line in college football).
After safeties, there is a free kick instead of a kickoff. A free kick is made from a team's own 20-yard-line and can be punted or placekicked.

Penalties

Rule violations are punished with penalties. Most penalties result in moving the football either towards the endzone in the case of a defensive penalty, or away from the endzone in the case of an offensive penalty. Some defensive penalties give the offense an automatic first down. In addition, if a penalty gives the offensive team enough yardage to gain a first down, the first down is automatically given. If a penalty occurs during a play, an official throws a yellow flag near the spot of the foul. When the play is over, the team that did not commit the penalty has the option of taking either the penalty or the result of the play. For example, say a defensive player commits an offsides penalty on first down by passing the line of scrimmage before the snap, and the offense gains eight yards on the play. The team with the ball has the option of taking the penalty and repeat the first down with five yards to go, or declining the penalty and scrimmaging with 2nd and 2.

Some common penalties

False start

A player on the offense, other than a back moving parallel to the line of scrimmage, moves just prior to the snap. Five yards.

Offsides

A player is on the wrong side of the ball at the start of a play. Five yards. Similar fouls: Touching an opponent before the snap is encroachment; lining up alongside the football instead of behind it is a neutral zone infraction.

Holding

A blocker unfairly impedes a would-be tackler or pass receiver, by grabbing the player's jersey, hooking, or tackling. Ten yards on offense or during a change of possession, five yards on defense and automatic first down. If the penalty occurred beyond the line of scrimmage, the penalty would be enforced from the spot of the foul.

Pass interference

After a pass is launched into the air, a defender pushes, hooks, grabs, or knocks down a would-be pass receiver, or if the receiver does the same to the defender to prevent an interception. First down at the spot of the foul if against the defense (15 yards from the previous spot in college football), or ten yards from the previous spot if against the offense. Similar penalties before a pass are called as holding or illegal contact.

Facemask

A player places his hand on an opponent's facemask during a play. Five yards, or fifteen (a personal foul) if the player hooks his fingers into the facemask or pulls on it.

Roughing the passer/kicker

A player places a hard hit on a passer long enough after a pass has been thrown to consider the contact avoidable, or places a hard hit on a punter or place kicker. Fifteen yards and automatic first down.

Running into the kicker

A lighter contact on a kicker, especially after the kick has been made. Five yards.

Intentional grounding

The passer throws a forward pass not near any eligible receiver, without first leaving the area behind where the blocking linemen were standing before the snap (the "pocket"), or the passer throws a forward pass outside of the pocket which does not reach the original line of scrimmage and is not near any eligible receiver. Ten yards plus loss of down, except if the penalty occurred in the end zone, then it is ruled a safety, and the defense is awarded 2 points. In college football and high school football, the defense is also credited with a quarterback sack. Note that spiking the ball to stop the clock is exempt from this.

Ineligible receiver downfield

On every play the offense must have 7 players on the line of scrimmage, the player furthest from the ball on each side are eligible receivers; the interior five players are considered ineligible to receive passes. This penalty is called if one of the 5 interior players is more than five yards past the line of scrimmage during a forward pass.

Dead ball personal foul

After the play is blown dead, a player tackles or makes rough contact with a player on the other team. Fifteen yards, automatic first down if on defense.

Unnecessary roughness

A catch-all for rough play that doesn't merit its own foul. An example is an avoidable late hit on a ball carrier who has run out of bounds. Fifteen yards.

Unsportsmanlike conduct

Another catch-all call, commonly used for taunting, excessive celebration after a touchdown, and certain banned forms of pantomime (like slashing the throat). Fifteen yards.

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