What is the "number system" for haircuts

[asolof]

Frequently there are posts referring to a haircut with something like "a number 2 guard". I assume this relates to the length of the trimmer attachment and therefore the length of the resulting haircut. Does #1 = 1 inch of hair and #2 = 2" of hair, etc.? If not, what lengths do the different numbers correspond to?

 

[markymark123]

someone correct me if im wrong, but #1 i believe is 1/8 inch, #2 is 1/4 inch, #3 3/8 inch, and on and on going up by 1/8 inch