Well, that would depend whether you're French or English. Lets stick with the French.
Ligne (pronounced 'lin'), is an archaic unit of measurement commonly
used in French and Swiss horology dating back to the 1700s. It is a
pre-decimal unit of measurement. Basically, 1 ligne is 2.2558mm.
Watchmakers made movements in lignes where the average size was
around 19 lignes (or just under 43mm in diameter). As continental
Europeans moved to the decimal system, Swiss and French watchmakers
refused to budge. So even today, measurements in lignes are often quoted
in relation to the watch movement diameter.
Lignes as a unit of measurement is easily distinguished by metric and
imperial units with the number followed by 3 primes (e.g. 18'''). For
students of horology: the movement diameter is measured at the outside diameter of the bridges, not at the dial/casing flange.
Novice watch enthusiasts sometimes confuse lignes with calibre or what Americans call size.
For example, the Omega movement mentioned yesterday was calibre 19LBN,
happens to be exactly 19 lignes in diameter. But this a pure
coincidence. American pocket watch movements are referred to by sizes.
For example, size 10 is 1.500 inches or 38.10mm or converted in lignes,
16 and 7/8s. Clearly, the relationship between lignes, millimetres,
inches and sizes is not 'blindly obvious' and to covert between we need
help of tables.
To this day, the French and Swiss stick with lignes and Americans with sizes, and neither side is willing to adopt the logical decimal unit of measurement.
Being Australian watchmakers ourselves, we
simply state that our NH2 movement is 36.60 mm in diameter. Which would
make it just over 16 lignes and just under size 8.