Watchmakers' Use
There are 12 lignes to one French inch (pouce). The standardized conversion for a ligne is 2.2558291 mm (1 mm = 0.443296 ligne), and it is abbreviated with the letter L or represented by the triple prime, ‴.
This is comparable in size to the British measurement called "line" (one-twelfth of an English inch), used prior to 1824.
Read more about this topic: Ligne