Thumb

The width of a thumb at the knuckle, taken as roughly one inch, and the body measure from which the inch descends.

A thumb is very nearly an inch, and that is the whole point of it. An adult thumb measures about two to two and a third centimetres across the knuckle, which is a little under an inch for most people, and the top joint from tip to first knuckle runs close to a full inch. Neither is exact, and they differ by several millimetres from one hand to the next, which is precisely why the thumb was eventually replaced in law by something countable: Edward II’s three barleycorns, laid end to end, in 1324. Several European languages never bothered separating the two ideas at all. The Dutch duim, the Swedish tum and the French pouce all mean thumb and inch together, and English kept the relationship even after it split the words. The measure is still in genuine use. The Royal Navy teaches a rule of thumb in damage control shoring, measured from thumbnail to hand, to set the angle of a shore against a breast; carpenters and bricklayers reach for a thumb as a quick inch constantly, because it is attached and a tape measure is in the van. The folk etymology claiming the phrase refers to a legal limit on the thickness of a stick for beating one’s wife is an eighteenth-century satire later mistaken for fact. No such law existed.