In horology, a movement is the internal mechanism of a clock or watch, as opposed to the case, which encloses and protects the movement, and the face which displays the time. The term originated with mechanical timepieces, whose movements are made of many moving parts. It is less frequently applied to modern electronic or quartz timepieces, where the word module is often used instead.

In modern mass produced clocks and watches, the same movement is often put into many different styles of case. When buying a quality pocketwatch from the mid-19th to the mid-20th century, for example, the customer would separately pick out the movement and the case of his choice, and the movement would be installed in the case for him. Mechanical movements get dirty and the lubricants dry up, so they must periodically be disassembled, cleaned, and lubricated.

Most movements contain four primary components: a power source, wheel train, escapement, and an escapement. The power is usually derived from a either a mainspring, or a weight suspended from a cord wrapped around a pulley. The wheel train that transmits the force of the power source to the escapement. Large gears known as wheels mesh with small gears known as pinions. A separate set of wheels, the motion work, divides the motion of the minute hand by 12 to move the hour hand, and in watches another set, the keyless work, allows the hands to be set.

The escapement is a mechanism that allows the wheel train to advance, or escape, a fixed amount with each swing of the balance wheel or pendulum. It consists of a gear called an escape wheel, which is released one tooth at a time by a lever that rocks back and forth. Each time the escape wheel moves forward it also gives the pendulum or balance wheel a push to keep it moving. The timekeeping element is called an oscillator, and is typically either a pendulum or a balance wheel. It swings back and forth, with a precisely constant time interval between each swing, called the beat.

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