Caliber (or Calibre) is the internal mechanism or movement of a watch. It is a crucial part of the watch’s construction, and determines the accuracy, power reserve and other features of the watch. The caliber is composed of hundreds of tiny parts, including several gears, springs, and jewels, all of which work together to keep time. The caliber of a watch can range from simple, low-cost quartz models to complex, high-end mechanical watches. Caliber is also used to refer to the size of the watch, typically measured in millimeters or fractions of an inch.
