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analog31 4 hours ago  [ - ]

I think that both the Celsius and Fahrenheit scales are now defined in terms of Kelvin and a formula. Most if not all of the units of measure are no longer related to their history except for an agreement to make the new units match the old ones at some point.

0cf8612b2e1e 3 hours ago  [ - ]

The US is technically metric in that the units are defined in terms of the metric equivalent. A US inch is exactly 2.54cm.

mikewarot an hour ago  [ - ]

You can thank Carl Edvard Johansson[1] for that, the inventor of the gage block.[2]

He was the one who decided to make it a convenient value, according to Wikipedia[3]

  As a result of the definitions above, the U.S. inch was effectively defined as 25.4000508 mm (with a reference temperature of 68 degrees Fahrenheit) and the UK inch at 25.399977 mm (with a reference temperature of 62 degrees Fahrenheit). When Carl Edvard Johansson started manufacturing gauge blocks in inch sizes in 1912, Johansson's compromise was to manufacture gauge blocks with a nominal size of 25.4mm, with a reference temperature of 20 degrees Celsius, accurate to within a few parts per million of both official definitions. Because Johansson's blocks were so popular, his blocks became the de facto standard for manufacturers internationally, with other manufacturers of gauge blocks following Johansson's definition by producing blocks designed to be equivalent to his.
This makes it relatively easy to handle making metric threads on imperial machines, and vice versa, because you can use 50 / 127 gear tooth pairs to do the ratios.

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

[2] https://en.wikipedia.org/wiki/Gauge_block

[3] https://en.wikipedia.org/wiki/Inch#History