Chess Clock

a clock for recording the time spent by each chess player in considering his next move during a game of prescribed duration.

A chess clock usually consists of two clock movements in a single case with two faces and two hands. The two movements are kinematically connected in such a way that at any given moment only one clock is running. Pushing the buttons located in the upper part of the case starts one clock and simultaneously stops the other. After making his move, the player pushes the button, stopping his own clock and starting his opponent’s clock. Thus the total playing time is calculated separately for each player. Two to three minutes before the prescribed playing time elapses, the hand of the clock raises a flag. As soon as the hand reaches the numeral 12, the flag falls, signaling that the playing time has expired.

Chess clocks are sometimes equipped to indicate how many times a button has been pushed. Some clocks consist of a single continuously running movement and hands that are switched on and off by pushing a button. Another type of clock has a separate case for each movement. Chess clocks are usually accurate to 60 sec per 24 hr. Digital electronic chess clocks are now being developed (1978).

Mechanical chess clocks were introduced in Great Britain in the mid-19th century, until which time sandglasses were used. They were first used in an international tournament in 1883, in London.