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.
Quarterback
As late as the 1960s, running plays occurred more frequently than passes. NFL quarterback Milt Plum later stated that during his career (1957-1969) passes typically only occurred on third downs and sometimes on first downs. Quarterbacks only increased in importance as rules changed to favor passing and higher scoring and as football gained popularity on television after the 1958 NFL Championship Game, often referred to as "The Greatest Game Ever Played". Early modern offenses evolved around the quarterback as a passing threat, boosted by rules changes in 1978 and 1979 that made it a penalty for defensive backs to interfere with receivers downfield and allowed offensive linemen to pass-block using their arms and open hands; the rules had limited them to blocking with their hands held to their chests. Average passing yards per game rose from 283.3 in 1977 to 408.7 in 1979.
The NFL continues to be a pass-heavy league, in part due to further rule changes that prescribed harsher penalties for hitting the quarterback and for hitting defenseless receivers as they awaited passes. Passing in wide-open offenses has also been an emphasis at the high school and college levels, and professional coaches have devised schemes to fit the talents of new generations of quarterbacks.