You have seen button cards with the sizes indicated in inches and millimeters. No doubt you have seen the letter L followed by a number or the word Ligne. What in the world is a ligne?
In French or German, Ligne means line. It is a unit of measurement applied to the diameter of buttons that predates metric, which was installed in the 18th century as the universal standard of measurement in Europe. In the 9th century, German button makers used ligne to represent a measurement of 1/40th of an inch, more or less. Today, Ligne, a term probably brought over to the US from German immigrants, is the standard measurement for buttons. Strict and precise, a ligne equals exactly 1/40th of an inch of the diameter of a button or a snap.
This snappy button sizing chart from Britex Fabrics in San Francisco is available as a download HERE. But do visit their website if you are looking for high quality fabrics, notions and trims. They have great sales too!! Follow their BLOG to keep up to date.