Ligne, or line, is a unit of measurement that was used for centuries prior to the adoption of metric system, to define the diameter of buttons, watch movements and in the manufacture of ribbon. Even in the modern world button sizes are still often defined by their ligne size with 1 ligne approximately equal to 1/40th of an inch. Some confusion still remains, however, as some countries calculate ligne measurements in different ways, for example in France, 1 inch is equal to 11 ligne.
The accepted way to calculate the ligne of a button is to divide the button diameter in millimetres by 0.635.
To keep things simple, we put together this simple chart to convert button sizes from ligne to mm, and vice versa.