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.
Canadian football
During the last three minutes of a half, the penalty for failure to place the ball in play within the 20-second play clock, known as "time count" (this foul is known as "delay of game" in American football), is dramatically different from during the first 27 minutes. Instead of the penalty being 5 yards with the down repeated, the base penalty (except during convert attempts) becomes loss of down on first or second down, and 10 yards on third down with the down repeated. In addition, as noted previously, the referee can give possession to the defence for repeated deliberate time count violations on third down.