|A time control is imposed on the tournament play of almost all two-player board games to ensure that neither player delays the game or gains an unfair advantage by thinking for an unduly long time. Time controls are typically enforced by means of a game clock.|
In tournament chess the most common time controls for professionals are at 40/2, SD/1 time controls, which means that the first 40 moves must be made in two hours and then all remaining moves be made in an hour. Any extra time from the original two hours carries over to the sudden death time. In an effort to popularize chess as a spectator sport, an increasing number of tournament games are played at SD/30, which means that each player has thirty minutes to complete all of his moves. This time control is referred to as Rapid Chess. Recreational games are often played at SD/5, which is referred to as "blitz" chess.
In tournament Scrabble the time control is standardized to 25 minutes per side with a 10-point penalty for each minute that is used in excess. The number of extra minutes is rounded up, so that overstepping time control by 61 seconds carries a 20-point penalty.
In tournament Shogi and Go, players generally have a set limit for all of their moves, after which the byoyomi rule goes into effect. When a player runs out of time, he does not automatically lose as in chess, but has a limited time for each remaining move. For example, in fairly fast shogi games, a game will be played with 30 minutes per side with a 30 second byoyomi. This means each player will have 30 minutes for their moves, after which they must make each move within 30 seconds. If a player uses less than 30 seconds on a move during byoyomi, the additional seconds are not carried forward.
Bobby Fischer proposed (and attempted to patent) a time control concept similar to byoyomi, but which would be in effect from the start of the game. Each player is allowed a large block of time (say 30 minutes), which should be used on complicated moves, and a small amount of time (say 30 seconds) on each move before the main bank would begin to decline. Moves made in less than 30 seconds would not cause the bank to increase. This method not only prevents time scrambles in which moves must be played almost thoughtlessly, it also encourages players not to move quickly even when they have a good deal of time remaining, because there is no advantage to moving in 2 seconds rather than 29.
Top-line digital game clocks are capable of accommodating all of the above time controls, as well as many others.