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.
NFL preseason
The exhibition season typically begins the first weekend of August with the Hall of Fame Game; though in some years it can be on the second weekend. Previous seasons have seen the American Bowl game held the last weekend of July. The first full schedule of exhibition games is held the following weekend. Most games are held on Thursday, Friday, or Saturday nights, with one nationally televised game each night of the week: NFL Network airs a Thursday game, CBS and Fox a Friday and Saturday night game each, NBC with Sunday night games, and ESPN a Monday night game. Unlike the regular season, CBS's and Fox's national exhibition game opponents are selected regardless of conference. Four full weekends of games are held. The fourth and final full week of exhibition games (fifth weekend overall) usually has teams playing exclusively on Thursday night (plus Friday, if any), with no national games. This allows teams a few extra days to prepare for the first week of the regular season. It also prevents conflict with the start of the regular seasons for high school and college football, allowing those venues to expand their first weekends' games from Thursday through Monday (Labor Day).