A chess clock consists of two adjacent clocks with buttons to stop one clock while starting the other, so that the two clocks never run simultaneously. Chess clocks are used in chess and other two-player games where the players move in turn. The purpose is to keep track of the total time each player takes for his or her own moves, and ensure that neither player overly delays the game.
Chess clocks were first used extensively in tournament chess, and are often called game clocks. In a tournament, the arbiter typically places all clocks in the same orientation, so that he can easily assess games that need attention at later stages. Their use has since spread to tournament Scrabble, shogi, go, and nearly every competitive two-player board game, as well as other types of games. The first time that game clocks were used in a chess tournament was in the London 1883 tournament.
The simplest time control is "sudden death", in which players must make a predetermined number of moves in a certain amount of time or forfeit the game immediately. A particularly popular variant in informal play is blitz chess, in which each player is given a short time (e.g. five minutes) on the clock in which to play the entire game.
The players may take more or less time over any individual move. The opening moves in chess are often played quickly due to their familiarity, which leaves the players more time to consider more complex and unfamiliar positions later. It is not unusual in slow chess games for a player to leave the table, but the clock of the absent player continues to run if it is his turn, or starts to run if his opponent makes a move.