Gameplay in American football consists of a series of downs, individual plays of short duration, outside of which the ball is or is not in play. These can be plays from scrimmage – passes, runs, punts or field goal attempts (from either a place kick or a drop kick) – or free kicks such as kickoffs and fair catch kicks. Substitutions can be made between downs, which allows for a great deal of specialization as coaches choose the players best suited for each particular situation. During a play, each team should have no more than 11 players on the field, and each of them has specific tasks assigned for that specific play.
The objective of this game is to score more points than the other team during the allotted time.[1] The team with the ball (the offense) has 4 plays (downs) to advance at least 10 yards, and can score points once they reach the opposite end of the field, which is home to a scoring zone called the end zone, as well as the goalposts. If the offense succeeds in advancing at least 10 yards, they earn a "first down" and the number of tries allotted is reset and then they are again given 4 tries to advance an additional 10 yards, starting from the spot to which they last advanced. If the offense does not advance at least 10 yards during their 4 downs, the team without the ball (the defense) regains control of the ball (called turnover on downs).
On offense, points are scored by advancing the ball into the opponent's end zone for a touchdown (worth six points), or by kicking the ball from the playing field through the raised vertical posts (the goalposts) which are most commonly situated on the end line of the end zone for a field goal (worth three points). After scoring a touchdown, the offense is given an additional opportunity from the 2-yard line (3-yard line in amateur football) to attempt to score (in the NFL, 15-yard line on 1-point conversions). Conversion attempts are used to score 1 or 2 points as follows:
While the opposing team has possession, the defense attempts to prevent the offense from advancing the ball and scoring. If an offensive player loses the ball during play (a fumble) or the ball is caught by a defensive player while still in the air (an interception), the defense may attempt to run into the offense's end zone for a touchdown. The defense may also score points by tackling the ball carrier in the offense's own end zone, called a safety (which is worth two points).
Collegiate and professional football games are 1 hour long, divided into four quarters of 15 minutes each. In high school football, 12 minute quarters are usually played. The clock is stopped frequently, however, with the result that a typical college or professional game can exceed three hours in duration. The referee controls the game clock and stops the clock after any incomplete pass or any play that ends out of bounds. In addition, each team is allowed 3 timeouts in each half that they may use at their own discretion. The clock normally runs during the action of plays, with a few exceptions known as untimed plays. Some high schools employ a mercy rule in which the clock runs continuously after one team's lead over the other achieves a certain number of points. In these instances, the clock only stops for injuries, or time outs called by a team or a referee.
The clock may also be stopped for an officials' time-out, after which, if the clock was running, it is restarted. For example: if there is a question whether or not a team has moved the ball far enough for a first down, the officials may use a measuring device (the chains) to determine the distance. While this measurement is taking place, the officials will signal for a stoppage of the clock. Once the measurement is finished and the ball is placed at the proper location (spotted), the referee will then signal for the clock to restart. Additional situations where officials may take a time-out are to administer a penalty or for an injured player to be removed from the field.