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
If a game is tied at the end of four quarters, "overtime" is played. In overtime, the coin is tossed to determine which team will possess the ball first. The winner of the coin toss can choose to give the ball or receive the ball. If the first possession results in a field goal, the other team is given possession to match or better the field goal; therefore continuing the game. If the first possession results in a touchdown or safety, the scoring team wins. During the regular season in the NFL, one overtime period is played (with each team receiving two time outs). If both teams are tied after the 10-minute overtime, the game officially ends in a tie. In the playoffs, 15-minute overtime periods continue until a winner is determined. Overtime follows a three-minute intermission after the end of the regulation game. Prior to start of overtime, a coin flip is performed in which the captain of the visiting team calls the toss. The team that wins the coin flip has the option either to receive the kickoff or choose the side of the field they wish to defend. Ties are rare in the NFL; a game between the Minnesota Vikings and Green Bay Packers on September 16, 2018 ended in a 29–all draw. The previous occurrence was one week earlier, on September 9, 2018.