Coins left on headstones of military personnel are messages from someone who has visited the cemetery; each coin denomination denotes a specific message. For example, a penny left on a grave denotes that someone visited the grave, while a nickel indicates that the deceased and the visitor trained at the same boot camp.
A dime left on the grave stone means that the visitor and the deceased served together. Similarly, a quarter left on the grave signifies that the visitor was present when the deceased was killed in combat or elsewhere. This practice has its roots during the Vietnam era, when leaving coins was a way of saying that they would buy the deceased a beer when the deceased and the visitor met again in the future.