There are also clocks for the blind that have displays that can be read by using the sense of touch. For convenience, distance, telephony or blindness, auditory clocks present the time as sounds. Most digital clocks use electronic mechanisms and LCD, LED, or VFD displays. Two numeric display formats are commonly used on digital clocks: 24-hour notation and 12-hour notation. Digital clocks display a numeric representation of time. Analog clocks usually indicate time using angles. This object can be a pendulum, a tuning fork, a quartz crystal, or the vibration of electrons in atoms as they emit microwaves. The timekeeping element in every modern clock is a harmonic oscillator, a physical object (resonator) that vibrates or oscillates at a particular frequency. The development of electronics in the 20th century led to clocks with no clockwork parts at all. A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation. The next development in accuracy occurred after 1656 with the invention of the pendulum clock. During the 15th and 16th centuries, clockmaking flourished. Spring-driven clocks appeared during the 15th century. 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 balance wheels. Water clocks, along with the sundials, are possibly the oldest time-measuring instruments. There is a range of duration timers, a well-known example being the hourglass. A sundial shows the time by displaying the position of a shadow on a flat surface. Devices operating on several physical processes have been used over the millennia. The clock is one of the oldest human inventions, meeting the need to consistently measure intervals of time shorter than the natural units: the day, the lunar month, and the year. Watches and other timepieces that can be carried on one's person are often distinguished from clocks. In general usage today a ' clock' refers to any device for measuring and displaying the time. A silent instrument missing such a striking mechanism has traditionally been known as a timepiece. The word clock is derived (via Dutch, Northern French, and Medieval Latin) from the Celtic words clagan and clocca meaning 'bell'. A clock is an instrument to measure, keep, and indicate time.

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