Clocks and watches are used to measure and display time. The clockwork is the central component of mechanical clocks. Chronometers are watches whose mechanics can be proven to have a certain running accuracy. Radio-controlled clocks are regularly synchronised with the DCF 77 time signal transmitter. Early elementary clocks as well as optical numerical displays, on the other hand, function without moving parts. Many variants are available, from wristwatches to wall clocks, alarm clocks, kitchen clocks in the cooker to electric clock displays.