Continuing our series on the Laws of Chess, we now look at the rules surrounding the use of clocks...
Placing the clock
Who decides where the clock should be placed? In league matches, it is normal practice for the clock to be placed on White's left-hand side. If White is left-handed, he may prefer to have the clock on his right-hand side; he may suggest that to his opponent. But whilst this may be acceptable in a league match, where there is no arbiter, it will not be allowed in a tournament, where it's the arbiter who decides (FIDE Rule 6.4). The arbiter may wish to walk down a central aisle to view clocks on either side, and this may require some of the clocks to be placed on White's right-hand side. So if you're playing in a tournament, don't move the clock to the opposite side, even if your opponent suggests it.
Using the clock
The clock must be pressed by the same hand that made the move (Rule 6.7b). Forcible presses, or picking up the clock, are not allowed, and can be punished by the arbiter, who might initially warn the player, but can penalise him by awarding his opponent more time (Rule 6.7c). Ultimately, the game could be declared lost if the problem continues.
A move is not considered to have been completed until the clock has been pressed. The only exceptions to this are when the move achieves checkmate or stalemate. Checkmate and stalemate end the game immediately, so in a time scramble that ends in mate, there's no need to slam the clock after making the decisive move (Rule 6.7a).
Who can call a flag fall? Certainly not a spectator, or another player. Unfortunately, if this happens, there's nothing that can be done about it, other than to punish the culprit if this happens during a tournament by banning the person from the event. So, other than the opponent, can anyone else call flag-fall?
It may surprise some people to learn that an arbiter can call flag-fall, at least in a standard-play or rapidplay game (but not in Blitz). In fact, the arbiter should call flag-fall if he observes it (Rule 6.8).
Flag falls - who wins?
Not always the opponent, because the game will be drawn if it is impossible for the opponent to deliver checkmate by any series of legal moves (Rule 6.9).
Both flags down
If both flags have fallen, then on a digital clock you can see whose flag fell first. But with analogue clocks it is often impossible to establish whose flag fell first. In this case, the game is drawn if it happens in the final period of the game (when all remaining moves have to be made). But if it occurs in any period except the last one, then the game simply continues without penalty to either player (Rule 6.11).