Ligne (pronounced as “line”) is an old French unit of distance traditionally used to measure watch movements, buttons, and ribbons. Although watchmaking (especially in Europe) adopted the decimal metric system around the turn of the century, the use of lignes as a measure of the diameter of a watch movement continued in common usage and was often used to differentiate calibres. One ligne is equal to 1/12 of a traditional French inch (pouce), which is equivalent to 2.2559 mm or 0.08881 American inch.
Most common wristwatch movement measure between 7.75 ligne and 13.25 ligne (17.20 mm to 30.00 mm), though a few 16.5 ligne (37.22 mm) and larger movements are also found. 11.5 ligne (25.60 mm) is very common, with nearly every manufacturer producing a movement in this size since it represents a good compromise for most wristwatches.
Since the advent of computer-assisted design, watch movements are entirely based on the decimal metric system. Even historic movements that have been re-created (e.g. Montblanc’s Minerva movements) have been re-designed in millimeters rather than lignes. Nevertheless it is still common to see movement diameter specified in lignes in technical documentation.
The United States and Britain traditionally used standardized numbered sizes for pocket watch movements, ranging from size 0 (29.63 mm) to size 32 (56.73 mm) though these proved impractical for wristwatches. Smaller movements were measured below zero to 10/0 (22.01 mm). These were based on fractions of an inch with a denominator of 30, with size 6/0 being 1 inch (30/30) and size 25 being 2 inches (60/30). These units are archaic and are no longer seen apart from historical documentation.