Inch (in)
Modern equivalent
2.54 centimetres
A traditional length unit historically derived from the width of a human thumb.
The inch takes its name from the Latin uncia, a twelfth part, but it takes its size from the thumb. Dutch, Swedish and French all use a single word for both, and English kept the relationship even after separating the words. Edward II fixed it in 1324 as three dry, round grains of barley laid end to end — a definition that had the great advantage of being countable, where a thumb had to be argued about. That barleycorn still governs one thing: UK and US shoe sizes advance by exactly one barleycorn, a third of an inch, per size. The modern figure of exactly 25.4 millimetres was agreed internationally in 1959, which means the inch is now formally defined in metric terms — a quiet reversal, given that the metre was once the upstart.