Roman shoemakers had discovered that the length of three barleycorns equaled one inch.
So they used one kernel, or a third of an inch, as a measurement for shoe size.
In 1324, King Edward of England decreed that three barleycoms was indeed one inch.
In the seventeenth century, children’s shoe sizes were deemed to be less than, and adult sizes more than, thirteen barleycorns.
Size zero was a baby’s size, and the shoes went up in one-barleycorn increments to a children’s thirteen, after which adult’s sizes started again at one.
That calculation is still used to determine shoe sizes to this day.