American Football History and Rules

American football, referred to simply as football in the United States and Canada and also known as gridiron football, is a team sport played by two teams of eleven players on a rectangular field with goalposts at each end.

The offense team with possession of the oval-shaped football attempts to advance down the field by running with the ball or throwing it, while the defense team without possession of the ball, aims to stop the offense's advance and to take control of the ball for themselves. The offense must advance the ball at least ten yards in four downs or plays; if they fail, they turn over the football to the defense but if they succeed, they are given a new set of four downs to continue the drive.

Points are scored primarily 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 the game wins.

A football game is played between two teams of 11 players each. Playing with more on the field is punishable by a penalty. Teams may substitute any number of their players between downs.

Individual players in a football game must be designated with a uniform number between 0 and 99, though some teams may "retire" certain numbers, making them unavailable to players.

Duration and time stoppages

The football game last for a total of 60 minutes and are divided into two halves of 30 minutes and four quarters of 15 minutes. The two halves are separated by a halftime period, and the first and third quarters are followed by a short break. Before the game starts, the referee and each team's captain meet at midfield for a coin toss. The visiting team can call either "heads" or "tails"; the winner of the toss chooses whether to receive or kick off the ball or which goal they wish to defend. They can defer their choice until the second half. Unless the winning team decides to defer, the losing team chooses the option the winning team did not select—to receive, kick, or select a goal to defend to begin the second half.

Games last longer than their defined length due to play stoppage.

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; the safety kick follows a safety.

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