A clock is a device used to measure and indicate the passage of time. Different types of clocks include analog clocks, digital clocks, sundials, and atomic clocks. A watch is a type if clock that is usually worn on the wrist. An alarm clock is a special clock that is designed to wake people up in the morning. Note that most clocks commonly called atomic clocks are technically little more accurate than digital clocks. However, they are updated periodically by radio transmissions based on the time determined by a central atomic clock.