A chess clock is a timing device that assigns a separate, limited amount of thinking time to each player, counting down each one’s time alternately to ensure games are completed within a defined time frame.
In practice, a chess clock features two linked timers: when a player makes a move and presses the clock button, their own timer stops and their opponent’s starts. If a player runs out of time before the game ends, they lose immediately — unless the opponent has insufficient material to deliver checkmate, in which case the game is declared a draw.
In competitive play, the clock forces players to balance the quality of their thinking with the speed of their decisions. Spending too long in the opening or middlegame can lead to time trouble (zeitnot) in critical moments. A good habit is to check the clock regularly and adjust your pace before falling below five minutes on the timer.