They never did to begin with. US customary units descend from older English customary units. During the 19th century the British government redefined some things in some weights and measures acts and that was called the Imperial system because of the British Empire. US never used Imperial as they were happy doing their own independent shit.
US customary and Imperial units differed a bit until the 20th century. In the 30s there was an ‘industrial’ inch agreed upon, the 25.4mm as you said, but weights still differed. In the 50s there was a conference where the US, UK and some commonwealth countries agreed upon a standardised international yard and pound, the international yard being 0.9144 meters and the international pound 0.45359237 kilograms, defined in metric as you said. Liquid measures were not standardised for some reason so US and Imperial gallons still differ.
The formal definition of Fahrenheit is based on Kelvin these days.
They never did to begin with. US customary units descend from older English customary units. During the 19th century the British government redefined some things in some weights and measures acts and that was called the Imperial system because of the British Empire. US never used Imperial as they were happy doing their own independent shit.
US customary and Imperial units differed a bit until the 20th century. In the 30s there was an ‘industrial’ inch agreed upon, the 25.4mm as you said, but weights still differed. In the 50s there was a conference where the US, UK and some commonwealth countries agreed upon a standardised international yard and pound, the international yard being 0.9144 meters and the international pound 0.45359237 kilograms, defined in metric as you said. Liquid measures were not standardised for some reason so US and Imperial gallons still differ.
The formal definition of Fahrenheit is based on Kelvin these days.