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.
Scoreboard
In both the United States and Canadian football codes, the minimum details displayed are the time and score of both teams. A typical high school scoreboard will additionally display the down, the yardage of the line of scrimmage, the yards to go until a first down, the team with the possession (usually signified with the outline of a football in lights next to the possessing team's score) and the quarter. Higher levels will also include play clocks and the number of time outs left for each team. American football scoreboards may include a horn to signal the end of a quarter, but they are not used in larger venues. In those cases, the referee or public address announcer denotes the termination of a quarter vocally via the PA system; formerly quarter ends were denoted with the firing of a starting pistol in the era before digital timing.