Clock flag

A chess clock has two linked displays or dials, one for each player, and each side includes a flag: an indicator that a time period has ended. On a mechanical clock, which uses physical mechanisms and hands, it is usually a visible device; on a digital chess clock, which uses electronic displays, it appears as a screen signal.

The flag is the indicator. Flag fall is the rules event in which the allotted time expires. In competitive play, a flag is considered fallen when the arbiter, the official who applies the rules, observes it or when a player makes a valid claim.

Flag fall does not make every position an automatic loss. A player normally loses for failing to complete the required moves in time, but the result is a draw if the opponent could not deliver checkmate through any possible series of legal moves. Checkmate means that the king is attacked and has no legal response.