CALIBRE

In the past, the term calibre referred to the architecture of a movement — specifically the layout of the bridges on the mainplate — as well as its dimensions, whether the movement was round or shaped. Today, the term is used exclusively to define the movement’s diameter, which is still often measured in lignes, an old unit of measurement equivalent to 2.255 mm. For example, one may commonly refer to a calibre as 9 lignes (9’’’) or 12 lignes (12’’’).

Useful links:
– The movement
– Lengths and angles (units of measurement)