Chess clocks are specially designed for the game, as each players' time must be measured separately and accurately. Both clocks are set for a predetermined amount of time, in which each player must make a certain number of moves.
Essentially, the clocks are two standard stopwatches, except that their start/stop buttons are bound together in such a way that when one player presses his button, his clock will stop and his opponent's will start.
The players must make a predetermined number of moves in a certain amount of time, usually averaging around a minute or two a move. However, the players may take more or less time over any individual move, which means that whilst the start of the game is a relatively quick succession of moves, the players can spend a large period of time thinking over the more complex moves later. The players are permitted to leave the table during their opponents turn - but there is nothing stopping their opponent from deciding on their move and pressing the button to restart their opponents clock.