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.
Comparison of American and Canadian football
American football rules allow each team to have three timeouts in each half, and the NFL stops play for a "two-minute warning". However, NCAA football has no two-minute warning, but the clock stops on a first down until the ball is ready for play if the play ended in the field of play. In the CFL, each team has two time-outs a game but cannot use both in the last three minutes of the game, while at lower levels of Canadian football each team has two. In Canadian football there is a three-minute rather than a two-minute warning. Also, at all levels of Canadian football, the clock is stopped after every play during the last three minutes of each half. Once the referee has set the ball the clock will restart if the last play ended with a runner tackled in the field of play.