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.
Stance (American football)
In the National Football League (NFL), the average amount of plays per game is over 60 per team. This means that, in one game, a lineman playing the entire time could enter a stance more than 60 times. However, it is not likely to be the same stance every time. Furthermore, it is important to remember that the offensive team must remain still for one second prior to the ball being snapped. This means that once an offensive player has become set in a stance, he must stay in it until the ball is snapped and the play begins. Additional motion can result in a false start penalty. Defensive players, on the other hand, are permitted to shift as much as they want before a play. It is possible for a defensive lineman to line up in one stance and then change to another just before the play begins. He can even shift his body to another place.