The term originated with mechanical timepieces, whose clockwork movements are made of many moving parts.
When buying a quality pocketwatch from the mid-19th to the mid-20th century, for example, the customer would select a movement and case individually.
The movement is made of the following components:[2] Watch movements come in various shapes to fit different case styles, such as round, tonneau, rectangular, rectangular with cut corners, oval and baguette, and are measured in lignes, or in millimetres.
In horology, "caliber" refers to the specific internal mechanism of a watch or clock, also known as a movement.
Although the term originally was only used to refer to the size of a movement, it is now used to designate a specific model (although the same caliber can be used in many different watches or clocks).