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.
Mercy rule
At the middle- or high-school level, 34 states use a mercy rule that may involve a "continuous clock" (the clock continues to operate on most plays when the clock would normally stop, such as an incomplete pass) once a team has a certain lead (for example, 35 points) during the second half. That greatly decreases the amount of time taken for a game to complete, which reduces the leading team's chances to score more and the time that the trailing team must spend in facing an insurmountable deficit. In most states, the clock stops only for scores, timeouts (officials', injury, or charged), or the end of the quarter. Plays that would normally stop the clock, such as penalties, incomplete passes, going out of bounds, or change of possession, would not stop the clock. The rule varies by state; for example, the clock does not stop upon a score in Colorado, Kansas (regular-season games only), or Missouri (fourth quarter only).