The ligne (from the French word meaning "line") is a unit of length that was in use prior to the French adoption of the metric system in the late 1700s, and is still used by French and Swiss wristwatch makers to measure the size of a watch movement. In this usage there are 12 ligne 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 imperial inch).
In the ninth century the term ligne came into use among German button makers, where it was used to measure the diameter of buttons. The consensus definition was that a ligne was the measurement of a round wick, folded flat. In this sense it measures of an inch, but not exactly, for there were several inches in the kingdoms and petty states of Germany at that time. Such a measurement became the American measurement called "line", being one fortieth of the US-customary inch. Its use was again to measure buttons, and was probably introduced by German immigrants.