Comparison of American football and rugby union
A rugby union game is divided into two halves of 40 minutes (or shorter for lower-grade games, and breaks given halfway through each half if playing conditions are considered to be extreme) separated by a half time period of up to 15 minutes in an international match. Most notably, a rugby union game will continue after the scheduled end of a half (half-time or full-time) until the ball becomes dead - any occurrence that would have play restart with a scrum or lineout, or when a team scores. This has led to some "nail-biting" finishes where teams losing by only a small margin work their way towards scoring, and games can go on several minutes over time. Something similar can occur at the end of either half in American football, though usually for only a few seconds of additional time, because a play that begins before the half expires continues until the ball is dead regardless of whether there is any time left on the clock. In both sports, the clock is also stopped during substitutions and for injuries, so the referee does not need to add stoppage time as is done in soccer/Association football. American football games are made up of four quarters of 15 minutes each (less at the high school and youth levels), but the clock stops and starts according to specific rules, so that a 15-minute quarter lasts much longer.
Play clock
In all levels of Canadian football, the offensive team must run a play within 20 seconds of the referee whistling the play in; in amateur American football, teams have 25 seconds from the time the ball is declared ready for play. In the NFL, teams have 40 seconds timed from the end of the previous down. Before 2008, in college football, the play clock was 25 seconds after the ball was set, but the clock was not stopped for the ball to be set unless the previous play resulted in a stoppage of the clock. Now, the same intervals as the NFL are used, with minor differences for the final two minutes of each half. Various professional leagues have used their own standards; the XFL, for instance, used a 35-second play clock to encourage faster play. Arena football uses a 32-second play clock.