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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 their 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. The first time that game clocks were used in a chess tournament was in the London 1883 tournament. 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. In a tournament, the arbiter typically places all clocks in the same orientation, so that they can easily assess games that need attention at later stages.
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 their turn, or starts to run if their opponent makes a move.
Analog clocks are equipped with a "flag" that falls to indicate the exact moment the player's time has expired. Analog clocks use mechanical buttons. Pressing the button on one player's side physically stops the movement of that player's clock and releases the hold on the opponent's.
The drawbacks of the mechanical clocks include accuracy and matching of the two clocks, and matching of the indicators (flags) of time expiration. Additional time cannot easily be added for more complex time controls, especially those that call for an increment or delay on every move, such as some forms of byoyomi. However, a malfunctioning analog clock is a less serious event than a malfunctioning digital clock.
- Fischer (also known as increment)—before a player has made their move, a specified time increment is added to their clock. Time can be accumulated, so if the player moves within the delay period, their remaining time actually increases. For example, if the delay time is five seconds, and a player has four seconds left on their clock, as soon as their opponent moves, they receive the increment and has nine seconds to make a move. If they take two seconds to move, on the start of their next move they have twelve seconds. There is also a variant of this time control that adds the delay after a player makes their move (Fischer after), so the delay is added to the player's remaining time and is available for their next move. If however time runs out during their move, the game ends without the delay time being added. This variant prevents a player who is in time-trouble from taking advantage of the extra-time.
- Bronstein delay—with the Bronstein timing method, the increment is always added after the move. But unlike Fischer, not always the maximum increment is added. If a player expends more than the specified increment, then the entire increment is added to the player's clock. But if a player has moved faster than the time increment, only the exact amount of time expended by the player is added. For example, if the delay is five seconds, the player has ten seconds left in their clock before their turn and during their turn they spend three seconds, after they press the clock button to indicate the end of their turn, their clock increases by only three seconds (not five). This ensures that the time left on the clock can never increase, even if a player makes fast moves.
- Simple delay—when it becomes a player's turn to move, the clock waits for the delay period before starting to subtract from the player's remaining time. For example, if the delay is five seconds, the clock waits for five seconds before counting down. The time is not accumulated. If the player moves within the delay period, no time is subtracted from their remaining time. This time control is similar to a Bronstein with time added before the move.
- Overtime penalty—it is a sudden death time control, without any increment nor delay. The difference here is that when the time expires by dropping to zero, a flag is set, and the clock immediately starts counting up without limit. This time control applies to games where the amount of time used after the allowed time can be subtracted from the player's score as a penalty, such as Tournament or Club Scrabble.
- Hour Glass—a player loses in this time control when they allow the difference between both clocks to reach the specified total amount. For example, if the total is defined as one minute, both players start their clocks at thirty seconds. Every second the first player uses to think in their moves is subtracted from their clock and added to their opponent's clock. If they use thirty seconds to move, the difference between the clocks reaches one minute, and the time flag falls to indicate that they lose by time. If they have used twenty nine seconds and then push the clock's button, they have one second left on their clock and their opponent has fifty-nine seconds.
Keith Ammann (April 2012). "Winding Down: This year's rule changes may begin the last chapter in the history of the analog clock". Chess Life.
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