What is a mule in regards to coins?

The numismatic meaning of "mule" is an analogy to a cross-bred animal. Like the pairing of a horse and a donkey, a mule coin is generally a "hybrid" that has the design of one coin on its front side and a different coin on the other. The term's also used for paper money that's printed with a mismatched front and back.

Most mule pairings are accidental. They happen when the front and back dies for two different coins are put into the same coin press. When blanks enter the press they get one image on the front side and a non-matching image on the other.

There are two major types of mule coins:

(1) Denomination mismatches. These happen when dies for two different denominations are put into the press. A famous recent denomination mismatch happened when dies for the front of a Washington quarter and the back of a Sacajawea dollar were put into a press intended for dollars. These mules were jokingly called "62.5-cent pieces" because they were half dollar and half quarter, but they actually sold for thousands.

(2) Design mismatches. This occurs when dies for different designs of the same denomination are incorrectly mated. For example, if a new design is being struck sometimes one side of the older design is inadvertently re-used. There's an unconfirmed Lincoln cent mule that was produced when the Lincoln Memorial reverse design was introduced in 1959. A 1959-dated front was reportedly matched with the "wheat ear" design that had been discontinued in 1958.

Mules can also be variations and combinations of the above, including dies for coins of different countries being put in the same press. Some numismatists also use the term for coins that have the correct dies but are struck on blanks intended for other denominations such as a cent struck on a dime blank. Others consider that error to be a separate mistake called an "off-metal strike".

Modern mules are not very common because of strict quality control standards. For example most coin presses are now "keyed" so that each die will only fit its intended side and denomination.