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.
List of gridiron football rules
As in regular American football, each team usually has four downs per series. In order to achieve a series of downs, backyard football requires the team with the ball to complete two passes or reach a certain point on the field. Few games include enough people to run a chain crew to maintain the 10 yard familiar in most organized leagues. These structures encourages passing plays over running, as does the usual lack of offensive and defensive lines. Play continues until there is a turnover on downs (i.e. the offensive team fails to complete two passes in four downs), an interception occurs, or the team on offense scores a touchdown. Touchdowns are worth 6, 7, or 1 point(s) depending on the rules set out before the game.