Most people realise that the edge of a coin is milled or has reeding. However few people realise that this is a legacy of a long-ago war against forgers.
Old coins are examined for their edges. The edges of a hammered coin are often rough and uneven. But a milled coin has smooth, parallel reeding all the way around its circumference.
In the 1690s the silver coins of England were in a sorry state. Large numbers of them were being clipped to extract the silver from their edges. It was a major problem for the English economy, and it was one that Isaac Newton was able to do something about when he was appointed Warden of the Royal Mint in 1696.
To foil the clippers, Newton and his master, John Lockey, the Master of the Royal Mint, decided to put a series of marks along the edges of the coins. The marks on a hammered coin are always rough and irregular, but on a milled coin the marks are cut in parallel lines and are always even and regular.
If a milled coin is clipped, then the cutting is immediately obvious from the disturbance to the even edge.
Each country, each era even, of minting coins have their own method of milled edge markings. Many English silver coins were reeded i.e. milled with parallel vertical striations. For example, our current 50p coin has reeded edges. Some gold coins like the Guinea were milled with lettering. In the case of the gold Guinea the phrase DECUS ET TUTAMEN (An ornament and a safeguard) was used. Some coins, generally those of higher denominations were even grained in a coarse manner to the edges. Experienced coin auctioneers like https://www.hoskerhaynes.com/ will know what to look for when studying coins.
This is all important to numismatists. When a coin auctioneer is describing a piece in the auction catalogue, he or she will start by describing the edge before moving on to any other feature. The condition of the reeding on a very old coin can give a clue as to the amount of circulation the coin has had. A sharp, undisturbed edge on an old coin always makes a nice numismatic impression and is generally assumed to have been handled very little. On the other hand, a coin with soft reeding is likely to have been in circulation for a long time and is therefore likely to be of less interest to collectors. A mismatch between the date on the face of a coin and the edge is often indicative of a forgery or restrike.
The face of a coin is designed to be admired. The edge is designed to be impossible to lie about.
