Curiously, the inch is exactly 2.54 cm. The story goes that Henry Ford was frustrated at the lack of standardization of parts measurements and things; the US at that time used various different inches, while the cm was fixed. So Ford asked Swedish inventor Carl Edward Johansson to make some end-gauge for Ford's factories. Johansson was happy to, so while Congress mucked around trying to come up with a standard, he picked the handy 2.54 cm to an inch that we know today and delivered the blocks. The 2.54 conversion allowed a switch from metric to imperial threads in a lathe by using two wheels with 100 and 127 cogs on them. 2.5 cm would have worked too, but it was too disparate from the majority of inches. 2.54 was just close enough.