The term inch comes from the Latin term uncia applied to one twelfth of something, and specifically to one twelfth of a Roman foot.

The inch as a fraction of Edward I's yardEdit

It appears that the earliest official definition of the yard (then referred to as the ulna) was in a statute of King Edward I (1272-1307). This statute also defined its sub- and aggregated divisions, including the inch, in the following words:

"It is remembered that the Iron Ulna of our Lord the King contains three feet and no more; and the foot must contain twelve inches, measured by the correct measure of this kind of ulna; that is to say, one thirty-sixth part [of] the said ulna makes one inch, neither more nor less... It is ordained that three grains of barley, dry and round make an inch, twelve inches make a foot; three feet make an ulna; five and a half ulna makes a perch (rod); and forty perches in length and four perches in breadth make an acre."

It should be noted that the length of the inch was there defined in terms of the length of grains of barley, as well as one thirty-sixth of an ulna; one wonders which took precedence.