The Draper's Cloth Scale
Measuring cloth across a counter, with the customer watching.
Origin
Medieval in origin, with the ell and nail regulated by assize and the yard fixed by royal standard from the twelfth century.
The catch
A draper measured in nails of two and a quarter inches, a unit that exists for one reason: it is one sixteenth of a yard, so a length of cloth could be halved and halved again four times without ever reaching a fraction.
Three ells were in use across these islands and the Low Countries, and none agreed: the English at 45 inches, the Scots at 37 and the Flemish at 27. Contracts routinely specified which was meant. The yardstick behind the counter was checked against the local standard, and short measure in cloth was among the most commonly prosecuted trading offences.