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.
Comparison of American football and rugby union
In rugby, the kickoff to begin each half and restarts after scores are taken from the halfway line whereas in American football these can be taken from the 30, 35 or 40 yard line depending on if it is a college, NFL or high-school game. The yard lines of American football are vitally important during game play because a team's advance is measured against them which, in turn, determines possession of the ball. The 22 metre lines in rugby union effectively divide the field into approximate quarters (though not always as rugby pitches vary in size; the total length of the field of play must not exceed 100m but can be shorter than that if space is limited). The 22 metre lines determine the position from which "drop-outs" are taken and also mark the limit where a defending player may kick-the ball directly (without bouncing) into touch without losing the ground gained by the kick.