What are chess clocks and how are they used in tournaments?

A chess clock is a device with two timers on it. Each timer measures the amount of time remaining for one of the two players. A player's timer runs only during the player's turn to move. The time control in most scholastic tournaments is G/30 (each player has 30 minutes to make all the moves). This ensures that every game is finished in less than an hour. Not all boards will have a clock at the start of the round, but as the round goes on, clocks will be placed on the longer games. Players are encouraged to bring their own clocks and use them from the start of the game.