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.
Golden goal
The National Football League introduced sudden death during the regular season in the NFL in 1974 and had always had it in its playoffs. Like the NHL, the NFL's rule comes into use at the end of regulation of any regular season game. Until 2011, it applied for playoff games as well. A new "quarter" is started, with a kickoff. Whichever team scores first—either through a field goal or through a touchdown, or far more rarely a safety—wins the game and the game ends. Since in the preseason and regular season, teams are given a "fifth quarter" of 10 minutes to decide the game. Originally if neither team scored after 15 minutes the game ended in a tie. Since the season, each team gets one possession to score, unless one of them scores a touchdown or safety on its first possession. Sudden death rules apply if both teams have had their initial possession and the game remains tied. If after the OT period it remains tied during that time, the game still ends.