Calibre is a term dating back to the early eighteenth century at which time it referred to the dimensions and position of a watch’s components such as wheel’s, barrel and bridges etc. In modern parlance the term is used to denote a specific shape and type of movement, the origin and maker of the watch. 

A round calibre watch which is the most common refers to the casing and is described in terms of its diameter. Other types of calibre include the revolver calibre in which the barrel bridge resembles a pistol, curved bridge calibre and three quarter plate calibre. Tag Heuer use the term to designate individual watches for example a Tag Heuer Calibre 5 identifies a specific automatic watch with rapid date correction.