Where does shotgun gauge come from?

The term "gauge" for shotguns comes from the old practice of measuring the bore diameter of a shotgun barrel by the number of lead balls of the same diameter that could be made from one pound of lead. For instance, a 12-gauge shotgun has a bore diameter that can accommodate 12 lead balls, each made from 1/12 of a pound of lead. This system of measurement was established in England in the 18th century and has since been adopted worldwide for shotguns.