A clock is an instrument for measuring time and for measuring time intervals of less than a day. Those used for technical purposes, of very high accuracy, are usually called chronometers. The clock in its most common modern form (in use since at least the 14th century) displays the hours, minutes and, sometimes, seconds that pass during a twelve- or twenty-four-hour period.
The clock is one of the oldest human inventions, requiring a physical process that will proceed at a known rate and a way to gauge how long that process has run. As the seasons and the phases of the moon can be used to measure the passage of longer periods of time, shorter processes had to be used to measure off hours and minutes.
Clockmakers developed their art in various ways. Building smaller clocks was a technical challenge, as was improving accuracy and reliability. Clocks could be impressive showpieces to demonstrate skilled craftsmanship, or less expensive, mass-produced items for domestic use. The escapement in particular was an important factor affecting the clock’s accuracy, so many different mechanisms were tried.
Spring-driven clocks were developed during the 15th century, and this gave the clockmakers many new problems to solve, such as how to compensate for the changing power supplied as the spring unwound.
The first record of a minute hand on a clock is 1475, in the Almanus Manuscript of Brother Paul.
During the 15th and 16th centuries, clockmaking flourished, particularly in the metalworking towns of Nuremberg and Augsburg, and, in France, Blois. Some of the more basic table clocks have only one time-keeping hand, with the dial between the hour markers being divided into four equal parts making the clocks readable to the nearest 15 minutes. Other clocks were exhibitions of craftsmanship and skill, incorporating astronomical indicators and musical movements. The cross-beat escapement was developed in 1585 by Jobst Burgi, who also developed the remontoire. Burgi’s accurate clocks helped Tycho Brahe and Johannes Kepler to observe astronomical events with much greater precision than before.

The first record of a second hand on a clock is about 1560, on a clock now in the Fremersdorf collection. However, this clock could not have been accurate, and the second hand was probably for indicating that the clock was working.
Most types of clocks are built around some form of oscillator, an arrangement that goes through an endless sequence of periodic state changes, designed to provide a continuous and stable reference frequency. The periods of this oscillator are then counted and converted into the desired clock display.
* Mechanical clocks use a pendulum as their oscillator, which controls the rotation of a system of gears that drive the clock display.
* Electrical clocks use electrical current to run, rather than requiring manual winding and weights.
* Crystal clocks use an electronic quartz crystal oscillator and a frequency divider or counter. Most battery-powered crystal clocks use a 215 Hz = 32.768 kHz oscillator.
* Atomic clocks use a microwave oscillator (maser) tuned by the energy transitions of elements such as caesium, rubidium or hydrogen. These are the most precise clocks available. Atomic clocks based on caesium are used as the official definition of time today.
* Mains power clocks count the 50 or 60 hertz periods of their AC power.

* Radio clocks receive time signal broadcasts from a radio transmitter (which may be hundreds of kilometres away). The clock can decode the transmission and adjust its hands or display for perfect accuracy. The broadcast radio signals received are generated by an atomic clock. These clocks are used extensively by mariners, especially short-wave radio clocks which use simultaneous bursts of time-signals, often encoded or encrypted – not to be confused with number stations.