Tangentially - If it weren't for the arbitrary decision of Carl Johansson, converting lathes from metric to imperial units wouldn't be exact. Thanks to him, an inch == 2.54 cm exactly.[1]

This means you can use a 50 and 127 tooth gear pair to do conversions and make metric threads accurately on an imperial lathe, and vice versa.

[1] https://en.wikipedia.org/wiki/Carl_Edvard_Johansson#Johansso...