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.
American football rules
NFL Europa, a defunct league run by the NFL, used a 10-minute overtime period, with the constraint that each team must have the opportunity of possession; once both teams have had such an opportunity, the overtime proceeds in a manner similar to the NFL's. Thus, if Team A has the first possession of overtime and scores a touchdown and converts their kick (thus being 7 points ahead of Team B), Team A would then kick off to Team B (In the NFL, the game would have ended with the touchdown, without a conversion being attempted). Team B would have to match or exceed the 7 point difference within this ensuing possession; exceeding it would end the game immediately, while matching the difference would result in a kickoff to Team A. From this point, the overtime is sudden death. The defunct United Football League had also used this rule.