Coins have ridges (called “reeding” or “milling”) mainly because historically they helped prevent fraud by making it obvious if someone had shaved metal off the edges.

Original purpose

When many coins were made of precious metals like silver or gold, dishonest people would “clip” them by shaving tiny amounts off the edges and keeping the shavings, while spending the slightly smaller coin at full value. Adding ridges meant that any clipping would visibly damage or remove the pattern on the edge, so people could quickly spot tampered coins.

Modern reasons

Today’s everyday coins are mostly base metals, so clipping is no longer profitable, but ridges remain for a few reasons. They are a traditional design element, help deter counterfeiting, and make it easier—especially for visually impaired people—to distinguish coins like dimes and quarters from smooth‑edged pennies and nickels by touch.