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.
Overtime (sports)
High school rules vary depending on the state and conference, but most will have a sudden-death overtime procedure wherein the game ends upon scoring a golden goal, although in some instances the overtime will go until completion with the team in the lead after time expires (i.e., silver goal rules) declared the winner. The overtime period length may vary, but it is commonly 10 minutes long. Depending on the state, if the game is still tied at the end of the first overtime:The NFL introduced overtime for any divisional tiebreak games beginning in 1940, and for championship games beginning in 1946. The first postseason game to be played under these rules was the 1958 NFL Championship Game between the Baltimore Colts and New York Giants (the so-called "Greatest Game Ever Played").