Barleycorn

A barleycorn is a historical English unit of length equal to exactly 1/3 of an inch, defined (per the old convention) as the length of an actual dried barley grain. It's essentially obsolete everywhere except one surprising survivor: UK and US shoe sizing is still built on barleycorn increments, which is why shoe sizes step up in such oddly specific fractional amounts rather than a round number. It shows up in old woodworking and furniture-making texts occasionally as a unit of fine measurement, though no working shop measures in barleycorns today outside of historical curiosity.