Two-minute warning

In the National Football League, the two-minute warning is given when two minutes of game time remain on the game clock in each half of a game, i.e. near the end of the second and fourth quarters. There is an additional two-minute warning in the rare event only two minutes remain in an overtime period. If the football is in play when the clock reaches 2:00, the two-minute warning is called immediately after the play concludes, when the ball is declared dead. The two-minute warning stops the game clock in all cases.

History
The origins are from the days in the NFL when the official game time was kept by a member of the officiating crew, with the stadium clock being unofficial. Its purpose was a checkpoint to ensure that the teams knew how much time remained in the game. Since the late 1960s the NFL stadium clock has been official, but by then television was an important factor in the NFL, so the two-minute warning was retained as a commercial break and to serve as "tension building" time, and thus has become an important part of the game's flow.

Rules
In addition to those practical purposes, gradually, some rules have evolved that are unique to the final two minutes of each half and overtime. There is no special event at the ends of the first and third quarters, aside from swapping end zones, so there is no two-minute warning then, only at the halves.

The two-minute warning in the fourth quarter is an important milestone in the game for a team that is in the lead and looking to run out the clock. If the leading team has the ball on first down with less than two minutes to go in the game and the opposing team has no timeouts remaining, the quarterback can often safely end the game by "taking a knee" three times in a row without risking injuries or turnovers. This is because at the end of each play, the offensive team can take up to 40 seconds to start running the next play.

10-second runoff
The following situations result in a 10-second game clock runoff if the team in possession of the ball and is trailing or the game is tied and has no timeouts in that half remaining. The half or game can expire on this runoff.


 * Excessive timeouts due to injuries (see below)
 * One of the following six fouls is committed by the offense. Following the runoff, the game clock will resume again once the ball is set. The runoff can also occur if a team declines to use a timeout if it has timeouts remaining.
 * False start
 * Intentional grounding
 * Illegal forward pass beyond the line of scrimmage
 * Throwing a backwards pass out of bounds
 * Spiking or throwing the ball away after a down (unless after a touchdown)
 * Any other intentional act that causes the clock to stop

Injuries
If a player is injured and his team has timeouts in that half remaining, a timeout is automatically charged to that team to allow the injured player to be removed from the field. If a team is out of timeouts, they get an excessive "fourth timeout". However, any subsequent injuries after the fourth timeout result in a five-yard delay-of-game foul. In addition to an excessive timeout, there is a 10-second runoff (if it was an offensive player that was injured) or the play clock is reset to 40 seconds (if it was a defensive player).

Exceptions to the above include if the other team called a timeout immediately after the previous play to save time on the clock; the injury was caused by a foul by an opponent; or the previous play resulted in a change of possession, a successful field goal, or was a conversion attempt.

Other rules

 * Within the two-minute warning period in the 1st half of the game, any restarting of the game following a timeout will happen with a scrimmage down.
 * If the defensive team is trailing with no timeouts, a defensive foul with 40 or fewer seconds can be declined, allowing the half or game to end.
 * The half or game cannot end on a defensive foul unless it is declined by the offense.

Other football leagues
The CFL has a three-minute warning, whilst the arena football leagues have a one-minute warning.