Chaldron

The principal British coal measure, reckoned by volume in London and by weight on the Tyne.

The word comes from the Old French chauderon, a cauldron, and for four centuries the chaldron was how Britain bought and sold coal. It is also the clearest case in the whole archive of one name meaning two incompatible things at once.

London kept it as a volume: thirty-six bushels, heaped and struck according to rules the Corporation enforced with some severity at the wharves. Newcastle, which produced the coal rather than burning it, found volume useless for loading barges and came to reckon its chaldron by weight instead, at fifty-three hundredweight.

So the two cannot be converted into one another, and never could be. A Newcastle chaldron is roughly twice a London one, but only roughly, because the comparison runs between a measure of space and a measure of mass, and the answer depends on how the coal was broken and how it settled.

That ambiguity is what killed it. Parliament required coal to be sold by weight in the 1830s, and the chaldron gave way to the ton.