What is a “barleycorn” used to measure?

Shoe size in the United Kingdom, Ireland, India, and South Africa is based on the length of the ‘last’ (a last is a mechanical frame shaped as a human foot used by shoemakers in the manufacture and repair of shoes) These are measured in "barleycorn", which is an old Welsh/English unit that equates to 1⁄3 inch (8.47 mm). The notion of three barleycorns composing an inch appears as early as the 10th-century in the Welsh Laws of Hywel Dda.

This remains the basis for North American shoe sizes, with the main difference being that the U.S. and Canada start counting at 1 rather than at zero making a men's size 13 in the U.S. or Canada the same size as a men's 12 in the UK.

In much of Europe the shoe size is the length of the ‘last’, expressed in “Paris points” for both sexes and for adults and children alike. In this system a Paris point is 2⁄3 of a centimetre (6.67 mm).

The Mondopoint system, introduced by the ISO (International Standards Organisation)/is widely used in the sports industry to size athletic shoes, ski/skate boots, and pointe ballet shoes. It was also adopted as the primary shoe sizing system in the USSR/Russia, the former GDR, China, Japan, Taiwan and South Korea, and as an optional system in the United Kingdom, India, Mexico, and several European countries. The Mondopoint system is used by NATO and other military services.