A clock is an instrument to measure, keep, and indicate
time. The word clock is derived (via
Dutch, Northern French, and
Medieval Latin) from the
Celtic words clagan and clocca meaning "
bell". A silent instrument missing such a
striking mechanism has traditionally been known as a timepiece.
 In general usage today, a "clock" refers to any device for measuring and displaying the time.
Watches and other timepieces that can be carried on one's person are often distinguished from clocks.
The clock is one of the oldest human
inventions, meeting the need to measure intervals of time shorter than the natural units: the
lunar month, and the
year. Devices operating on several physical processes have been used over the millennia. A
sundial shows the time by displaying the position of a shadow on a flat surface. There is a range of duration timers, a well-known example being the
Water clocks, along with the sundials, are possibly the oldest time-measuring instruments. A major advance occurred with the invention of the
verge escapement, which made possible the first mechanical clocks around 1300 in
Europe, which kept time with oscillating timekeepers like
 Spring-driven clocks appeared during the 15th century. During the 15th and 16th centuries, clockmaking flourished. The next development in accuracy occurred after 1656 with the invention of the
pendulum clock. A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation. The
electric clock was patented in 1840. The development of
electronics in the 20th century led to clocks with no clockwork parts at all.
The timekeeping element in every modern clock is a
harmonic oscillator, a physical object (
resonator) that vibrates or
oscillates at a particular
 This object can be a
tuning fork, a
quartz crystal, or the vibration of
atoms as they emit
microwaves. Analog clocks usually indicate time using angles. Digital clocks display a numeric representation of time. Two numeric display formats are commonly used on
digital clocks: 24-hour notation and 12-hour notation. Most digital clocks use electronic mechanisms and
VFD displays. For convenience, distance, telephony or
blindness, auditory clocks present the time as sounds. There are also clocks for the blind that have displays that can be read by using the sense of touch. Some of these are similar to normal analog displays, but are constructed so the hands can be felt without damaging them. The evolution of the technology of clocks continues today. The study of timekeeping is known as