American football
Football games last for a total of 60 minutes in professional and college play and are divided into two-halves of 30 minutes and four-quarters of 15 minutes. High school football games are 48 minutes in length with two-halves of 24 minutes and four-quarters of 12 minutes. The two-halves are separated by a halftime period, and the first and third quarters are also followed by a short break. Prior to the start of the game, the referee and team captains for each team meet at midfield for a coin toss. The visiting team is allowed to call 'heads' or 'tails'; the winner of the toss is allowed to decide between choosing whether to receive or kick off the ball or choosing which goal they want to defend, but they can also defer their choice until the second half. The losing team, unless the winning team decides to defer, is allowed to choose the option the winning team did not select, and receives the option to receive, kick, or select a goal to defend to begin the second half. Most teams choose to receive or defer, because choosing to kick the ball to start the game would allow the other team to choose which goal to defend. Teams switch goals following the first and third quarters. If a down is in progress when a quarter ends, play continues until the down is completed.
Mercy rule
At the middle- or high-school level, 34 states use a mercy rule that may involve a "continuous clock" (the clock continues to operate on most plays when the clock would normally stop, such as an incomplete pass) once a team has a certain lead (for example, 35 points) during the second half. That greatly decreases the amount of time taken for a game to complete, which reduces the leading team's chances to score more and the time that the trailing team must spend in facing an insurmountable deficit. In most states, the clock stops only for scores, timeouts (officials', injury, or charged), or the end of the quarter. Plays that would normally stop the clock, such as penalties, incomplete passes, going out of bounds, or change of possession, would not stop the clock. The rule varies by state; for example, the clock does not stop upon a score in Colorado, Kansas (regular-season games only), or Missouri (fourth quarter only).