Nail
Modern equivalent
2.25 inches = 5.715 centimetres
The finest subdivision of the cloth trade, being two and a quarter inches, or one sixteenth of a yard.
The nail exists for one reason: it is a sixteenth of a yard, so a length of cloth could be halved, and halved again, four times over without ever arriving at a fraction. A draper folding fabric across a counter needed exactly that, and the customer watching needed to be able to check it. Four nails made a quarter and sixteen made the yard, and the unit was regulated by assize alongside the yardstick itself. The name is thought to come from the nail driven into the counter to mark the measure, though the evidence for that is thinner than the explanation deserves.