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.
Instant replay
American and Canadian football leagues vary in their application and use of instant replay review. In the National Football League, each coach is allowed two opportunities per game to make a "coach's challenge", and get a third challenge if both of the original two challenges were successful. A challenge can only be made on certain reviewable calls on plays that "begin" before the two-minute warning and only when a team has at least one time-out remaining in the half. The Canadian Football League uses similar rules as the NFL, except the game has a three-minute warning near the end of each half instead of two. In NCAA football, each team only has one challenge per game, and gets a second challenge if the first one is successful.