What it is
The ligne (often written L) is the button-trade unit for diameter. 1 ligne = 1/40 inch = 0.0635 cm. Not the old Paris typographic line, not Chinese cun, not Japanese sun.
How to use
Divide the centimeter diameter by 0.0635 to get ligne. 1.524 cm = 24 ligne. Reverse: ligne × 0.0635. The converter above uses this factor.
ligne = cm × 15.7480315
Example: 1 cm = 15.7480315 ligne
Common uses
Shirt buttons about 16-18 ligne (about 1.0-1.15 cm); suit front buttons often 24-28 ligne (about 1.52-1.78 cm); coats 32-36 ligne (about 2.03-2.29 cm). A 3/4-inch button is 30 ligne (1.905 cm).
Conversion tips
40 ligne = 1 inch = 2.54 cm. 16L ~ 1.02 cm, 18L ~ 1.14 cm, 24L = 1.524 cm, 30L = 1.905 cm. Do not mix ligne with denier.