Chess clock is a device used to measure time remaining for each player to complete his or her moves. Although originally created for chess, such a clock can be used in every two-player turn-based game.
The premise is simple: after a player makes a move, he presses the button located on his side of the clock. Pressing this button stops his clock and starts the clock of his opponent. Then the opponent makes his move and presses his button, which stops his clock and starts the first player’s clock again. The process is repeated until the game is finished or one of the players runs out of time.
There are two main types of chess clocks: analog and digital.
An analog clock consists of two clock faces and two buttons (one for each player). Each clock face contains a small plastic “flag” near the number 12. This flag rises as the hand approaches 12 and then falls to indicate that the player ran out of time. Because of this, chessplayers often use a verb “to flag”, which means the same as “to lose on time”.
A digital clock has electronic displays instead of clock faces. It is programmable, allowing for easy set up of different time controls. Digital chess clocks offer more accuracy than the analog ones, and display exact time remaining to the player.
Games are played with different “time controls”, from extremely fast to very slow. Some examples:
The “classical” one, currently used for World Championship matches and many top-level tournaments is as follows: “120 minutes for the first 40 moves, 60 minutes for the next 20 moves and then 15 minutes for the rest of the game plus an additional 30 seconds per move starting from move 61”.
The “FIDE” time control, used for many events organized by FIDE (Fédération Internationale des Échecs) and in many tournaments is “90 minutes for the first 40 moves followed by 30 minutes for the rest of the game with an addition of 30 seconds per move starting from move one”.
Time control for “rapid” World Championship is “25 minutes for the game plus 10 seconds per move”.
Time control for “blitz” World Championship is “3 minutes for the game plus 2 seconds per move”.
Time controls containing “increment”, which is an amount of time added to the player’s total time before every move, are only possible with digital clocks. Analog clocks have no such option.
Both types of clocks support “time odds”, which means that players start the game with a different amount of time. This is sometimes used to offset the strength difference between the players.
The clock is typically placed on the left side on the board (so white player has it on his left side, and black player on his right side). During the game, the clock must be handled with the same hand which is used for executing the moves. Moving the pieces with one hand while keeping the other one on the clock is not allowed. Also not allowed is pressing the button with excessive force.
When one of the players runs out of time, he loses the game. The only exception is running out of time when the opponent has no means to win on the board, in which case the game is declared drawn. For example, if white has king and queen and runs out of time, while black has only a lone king, the game is drawn.