Why Coins Were Milled

The ridges around the edge of a coin are a solution to a crime. When coins were worth the metal in them, a small shaving taken from the rim of each coin passing through a merchant's hands could be collected over months and melted into a saleable quantity, while the coin itself, being only a little lighter than it had been, still passed from hand to hand at face value. The loss was too small to notice on any single coin, and the practice was therefore hard to detect and impossible to prove against any particular person.

Marking the edge changed the problem rather than solving it directly. A coin whose rim carried a regular pattern of ridges could not be shaved without destroying the pattern, so the fraud became visible on the object itself rather than on a set of scales. The important point is not that the ridges made clipping harder but that they made it obvious, and a fraud that everyone can detect at a glance stops being worth attempting. The remedy worked by shifting the cost of vigilance from the state to the person taking the coin.