why do coins have ridges

Coins have ridges, also called reeding, primarily to prevent coin clipping—a practice where people shaved off small amounts of precious metals like gold or silver from the edges of coins to steal some metal without noticeably reducing the coin's size or weight. These ridges make it obvious when a coin has been tampered with. In addition to anti-clipping, ridges serve as a deterrent to counterfeiting by making coins harder to replicate accurately. They also help visually impaired people distinguish between different denominations by touch. Although today most coins are no longer made of precious metals, ridges have become a tradition and continue to serve these security and practical functions.