Also the British, we're metric on paper for most things but in practice we use both systems interchangeably for a lot of things. In a few cases like the roads and draught beer imperial units are mandated.
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Since this comes as a surprise to many of my first-time British colleagues: US customary and Imperial use the same names, but are different units. The US customary volume units (cups, gallons, etc.) are on two scales: the "tablespoon scale" which is all powers-of-two, and the "teaspoon scale" which is a third of some nearby tablespoon scale.
I used to have a handy chart of the mapping of "prefix" to power-of-two, for 2^-7 to 2^7.
Also, the US foot was supposed to be exactly 30cm, but the French couldn't get their shit together, in time.
Another fun fact is that UK gallons are based on the volume occupied by ten pounds of water. Combined with the fact there's 20 ounces in UK pint this means a fluid ounce of water weighs an ounce, and a pint of water weighs a pound and a quarter.
Not that you're very likely to encounter British fluid ounces any more, the smallest imperial unit of volume I generally run into is the half-pint.
Yeah. It'd've been neat if the US gallon was defined to be 256in^3 (with the US in being exactly 2.5cm). Then, it'd be exactly 4000 cm^3 (4L) to a US gallon, and the table of "prefixes" would be:
gallon 2^0
half-gallon 2^-1
quart 2^-2
pint 2^-3
cup 2^-4
gill 2^-5
jill 2^-6 -- invented half-gill
ounce 2^-7
tbsp 2^-8
half-tbsp 2^-9
dram 2^-10
peck 2^1
half-bushel 2^2
bushel 2^3
half-barrel 2^4
barrel 2^5
hogshead 2^6
??? 2^7
??? 2^8