a. According to the size of the button (ie, the size of the diameter)
That is, we often say 14#, 16#, 18#, 20#, 60# and so on! Its conversion formula is: diameter = model * 0.635 (mm). If we have a button in our hand, but don't know its size, we can use a caliper to measure its diameter (mm) and divide it by 0.635.
b. There is another way of expressing the size of the button, namely L
