Skillet,
The history goes back (1800's) to the British. I believe it was the Whitworth standard that started it all. That standard migrated to a British standard (maybe some of the folks across the pond can chime in here). That British standard was the basis for what has become our (our being the US standard of fine and course thread as we now know it) sometime in the early 1900's as we started to standardize sizes. If I am not mistaken they have a history that tracks back directly to the British Standard.
A very interesting question, one that I will be doing some research on, but would very much like to hear what others know.
Regards,
Glen |