What it is
The ligne (often written L) is the button-trade unit for diameter. 40 ligne = 1 inch exactly. Not the old Paris typographic line, not Chinese cun, not Japanese sun.
How to use
Multiply the inch diameter by 40 to get ligne. 0.6 in = 24 ligne; 3/4 in = 30 ligne. Reverse: ligne / 40. The converter above uses this exact factor.
ligne = in × 40
Example: 1 in = 40 ligne
Common uses
Shirt buttons about 16-18 ligne (0.4-0.45 in); suit front buttons often 24-28 ligne (0.6-0.7 in); coats 32-36 ligne (0.8-0.9 in). A 3/4-inch button is 30 ligne.
Conversion tips
40 ligne = 1 inch exactly. 16L=0.4 in, 18L=0.45 in, 24L=0.6 in, 30L=0.75 in. Do not mix ligne with denier.