How was the inch developed?

What we now know as the inch (from Latin uncia, or "12th part") was defined as 1/12 foot by the Romans. It was roughly a thumb's breadth, while a foot was roughly the length of a human foot. The Romans introduced the inch to Britain, where it was incorporated into the English system of weights and measures. The English made their own contribution to inch lore: In 1305, King Edward I decreed that an inch should be the measure of three dried barleycorns. (The Book of Answers, by Barbara Berliner)