[analog31]

An odd little factoid: A metalworking lathe will usually have a mechanism for cutting screw threads, with a multi-ratio gearbox for linking the rotation of the spindle to the motion of the carriage. For cutting metric threads, you have to change a pair of gears. So there will be a pair of gears with 50 and 127 teeth, which produces the 2.54 ratio. Then you have to look up the correct gearbox settings for each metric thread pitch in the manual. [0]

It's a big royal pain in the arse, and I've never had a machinist agree to turn metric threads on a lathe. They would rather wait a day and order a metric tap or die. Nowadays the CNC lathes just do it auto-magically.

Also, among old bicycles, there are some weird national thread standards that had metric diameters but inch thread pitches, because of machinery imported into countries like Italy and France during reconstruction after WWII.

[0] Edit: Just noticed duplicate comment preceding mine.