Basic Football Rules

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.
University of Southern California football games at Los Angeles Memorial Coliseum attract more than 90,000 fans.American football is played on a rectangular field 120 yards (110 meters) long by 53 1/3 yards (49 meters) 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 inbounds lines or 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 (5.6 m) apart (24 feet (7.3 m) in high school). The posts are connected by a crossbar 10 feet (3 m) from the ground.

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 almost all of the 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.

 Game duration
A standard football game consists of four 15-minute (typically 12 minutes in high school football) quarters, 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). 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 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, it loses possession of the ball.

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:

By running with the ball, also known as rushing. One ball-carrier can hand the ball to another; this is known as a handoff.
By throwing the ball to a teammate, known as a forward pass or as passing the football. 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, pitched, or tossed sideways or backwards at any time. This last type of pass is known as a lateral and is less common in American football than in rugby league or rugby union, where only backwards passes are permitted.
A down ends, and the ball becomes dead, after any of the following:

The player with the ball is forced to the ground (tackled) 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 most recent 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.
Officials blow a whistle to notify all 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 happens:

The team fails to get a first down— i.e., they fail to move the ball forward at least 10 yards in four downs. 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 posts to attempt a field goal.
When a defensive player catches a forward pass it is called an interception, and the player who makes the interception can run with the ball until he is tackled or forced out of bounds. After the intercepting player is tackled, forced out of bounds, or scores a touchdown, then 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 ballcarrier is tackled, forced out of bounds, loses the ball out of bounds, or commits certain penalties in his own end zone. This rare occurrence is called a safety.

 Scoring
A team scores points by the following plays:

A touchdown (TD) is worth 6 points. It is scored when a player runs the ball into or catches a pass in his opponent's end zone. A touchdown is analogous to a try in rugby with the major difference being that a try requires the player to place the ball on the ground.
After a touchdown, the scoring team attempts a conversion (which is also analogous to the conversion in rugby). 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 point after touchdown (PAT)), or run or pass it into the end zone in the manner of a touchdown for 2 points (a two-point conversion).
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 (extremely uncommon in the modern game, with only two successes in the last 60 years). A field goal is usually attempted on fourth down instead of a punt when the ball is close to the opponent's 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, fumbles the ball out of his end zone, or commits intentional grounding in his end zone. Additionally, if a punt is blocked by a defensive player, causing the ball to travel out the back of the end zone behind the offense, that too, is a safety (though this situation is significantly rarer). Certain penalties (primarily blocking fouls) 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 using a kicking tee from the team's own 30-yard line in the NFL or 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 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 the end zone. Punts and turnovers in the end zone can also end in touchbacks.

After safeties, the team that gave up the 2 points puts the ball into play with a punt or placekick from its own 20-yard line.