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.
High school football
Thirty-four states have a mercy rule that comes into play during one-sided games after a prescribed scoring margin is surpassed at halftime or any point thereafter. The type of mercy rule varies from state to state, with many using a "continuous clock" after the scoring margin is reached (wherein, except for specific situations, the clock keeps running on plays where the clock would normally stop), while other states end the game once the margin is reached or passed. For example, Texas uses a 45-point mercy rule (to stop the game) only in six-man football; for 11-man football there is no automatic stoppage but the coaches may mutually agree to use a continuous clock.