A ligne is a  measurement used in traditional watchmaking before the emergence of the metric system and is also known as the ligne parisienne.

It originates from the French foot (pied) in the Ancien Regime, during which time a foot became to be measured as 12 inches and each inch was equal to 12 lignes.  A ligne being equal to 2.26mm.

A ligne is signified by three apostrophes after the digit, for example a 10”’ movement measures 10 lignes which is equal to 22.6mm in diameter.