Time controls have historically been specified as some duration, plus some whole number of hours [often one] to be added after certain numbers of moves, because chess clocks were traditionally constructed by taking ordinary clock movements and adding mechanisms to selectively stop them, along with a flag that would fall when the minute hand reaches "12". If a time control is 2h40 plus one hour to be added after move 40, and a second hour after move 80, but no extra time beyond that, then both clocks would be set for 3:20 at the start of the game, and a player would run out of time if either:
- The flag falls when a player's clock reads 6:00 and fewer than 40 moves have been made.
- The flag falls when a player's clock reads 7:00 and 40-79 moves have been made.
- The flag falls when a player's clock reads 8:00.
Note that at no point in the game would the player's clock be adjusted to add the extra time. Instead, the players would simply observe what clock reading would represent the end of the game given the number of moves played thus far.
If clock makers had wanted to support adding 30 minutes at a time instead of 60, that would not have been difficult. All that would be needed would be to replace the minute hand with a double-ended version which would simultaneously point at e.g. 12:00 and 6:00 but still rotated once per hour. If enough moves have been played to extend a game by 30 minutes, the game would end when a player's flag falls and the hour hand is between the 6 and the 7. For a tournament to impose such a time control, however, it would be necessary to have all players' clocks support it.