The object of American football is to score more points than the opposing team within the time limit.
Field and players
American football is played on a field 360 by 160 feet (109.7 m ◊ 48.8 m). 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 (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.
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 parallel to the sidelines 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 goalposts (also called uprights) connected by a crossbar 10 feet (3.05 m) from the ground. The posts are, for high skill levels 222 inches (5.64 m) apart. For lower skill levels, these are widened to 280 inches (7.11 m).
Each team has 11 players on the field at a time. However, teams may substitute for any or all of their players, if time allows, during the break between plays. As a result, players have very specialized roles, and, sometimes (although rarely) almost all of the (at least) 46 active 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.
Start of halves
Similar to association football, the game begins with a coin toss to determine who will kick off to begin the games and which goal each team will defend. The options will be 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 will conduct the coin toss with the captains (or sometimes coaches) of the opposing teams. The team that wins the coin toss has three options:
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.
A standard football game consists of four 15-minute quarters (12-minute quarters in high-school football and often shorter at lower levels), with a half-time intermission 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), 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 an 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. 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 and are described in Overtime (sport).
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, to advance the ball 10 yards (9.1 m) towards their opponent's (the defense's) end zone. When the offense gains 10 yards, it gets a first down, which means the team has another set of four downs to gain yet another 10 yards or score with. If the offense fails to gain a first down (10 yards) after 4 downs, the other team gets possession of the ball at the spot of the football, beginning with their first down.
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 between his legs to a teammate, usually the quarterback.
Players can then advance the ball in two ways:
A down ends, and the ball becomes dead, after any of the following:
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:
A team scores points by the following plays:
Kickoffs and free kicks
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. A kickoff that goes out-of-bounds anywhere other than the end zone before being touched by the receiving team is considered an illegal procedure penalty, and the ball will be placed where it went out of bounds or 30 yards (27 m) from the kickoff spot, depending on which is more advantageous to the opposite team. Unlike with punts, once a kickoff goes 10 yards and the ball has hit the ground, it can be recovered by the kicking team. A team, especially one who is losing, can try to take advantage of this by attempting an onside kick. Punts and turnovers in the end zone can also end in a touchback.
After safeties, the team that gave up the points must free kick the ball to the other team from its own 20 yard line.
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. Conversely, some offensive penalties result in loss of a down (loss of the right to repeat the down). If a penalty gives the offensive team enough yardage to gain a first down, they get a first down, as usual.
If a foul occurs during a down, an official throws a yellow 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.
A few of the most-common fouls include:
Under certain circumstances clipping and blocking in the back are legal.