A chess clock consists of two clocks - one for each player - that are connected to each other in such a way that only one of them can run at any time. For analogue clocks, each clock has a "flag", which falls when the clock reaches the hour and which can therefore be used to show when a player has run out of time.
The time limit(s) may be decided in either of two ways. One option is to give each player a specified block of time for each block of moves, possibly with a time limit for the whole game; under this option, the time not used by a player during one period is added to the time he has available for the next period, either by turning back the clocks or by postponing the expiry time as shown on the clocks. The other option is the Time Delay Mode. Here, each player has a main thinking time, plus a fixed extra time for every move. At the start of a player's turn, the fixed extra time counts down first, with the main thinking time only counting down when the fixed extra time has expired. The amount of main thinking time that is not used is saved for the next move; however, the amount of fixed extra time is, by definition, the same for every move.
The arbiter decides where the chess clock is to be placed. At the time determined for the start of the game, the clock of the player playing white is started. If neither player is present at the start, the player playing white then loses all the time that elapses until he arrives unless the rules of the competition say otherwise. Any player who arrives more than one hour after the start of the game loses by default; again, the rules of the competition may say otherwise.
After each move, a player must press his clock button with the hand with which he made his move, thereby stopping his own clock and starting his opponent's. It is illegal to keep one's finger on or near the button. However, the button does not need to be pressed if the move that has just been made ends the game. If a player is unable to use the clock, an assistant may press the button for him.
A player who fails to make the required number of moves within the specified time limit loses the game; unless the opponent cannot possibly win, in which case the game is drawn. However, these points do not apply if the game was already won, or drawn by stalemate, by agreement, or by reaching a position from which neither side can possibly win, before it was noticed that one player had run out of time. If both flags are down and it is impossible to tell which fell first, the game is drawn if it happens during the period of the game in which all the remaining moves must be made; if it happens during any other period, the game continues.
Except at the end of the game, a player may only stop the clocks in order to seek the arbiter's assistance, for example if a pawn is to be promoted and the new piece is not available.
A chess clock with an evident defect must be replaced. The arbiter should use his best judgement to decide the times to be shown on the replacement clocks. The arbiter will also need to adjust the clock settings if an irregularity occurs and a previous position needs to be reinstated.
Screens, monitors and demonstration boards are allowed in the playing room. However, a player may not make a claim based solely on information shown on them.
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