Perhaps you noticed that United States dimes, quarters, half-dollars, and silver dollars have ridges, or grooves, around their edges. They were not put there for decoration, but had a very important purpose at one time in history.
During our country’s earlier years, all coins were made of gold or silver, and did not have ridges. Each coin’s value was based on the amount of gold or silver in it. For example, a $10 gold piece contained ten dollars worth of gold, and silver dimes contained ten cents worth of silver.
But some dishonest people sought to make an illegal profit from these coins. They filed off the edges and sold them for their value in gold or silver. The smaller-sized coin often went unnoticed, but this dishonest practice decreased the value of the original gold or silver coin.
To prevent this, the government began milling, or grooving, the edges so a coin could easily be identified if it was trimmed.
Coins today are no longer made of pure gold or silver, but the milled edges remain because people are accustomed to seeing them that way.
At the U.S. Mint in Philadelphia, one single stamping machine can produce 10,000 coins every minute of every hour of every day of the week!