Mule (MUL)
A mule is a coin struck from two dies that were never meant to be used together: the obverse of one coin and the reverse of another. Both dies are genuine Mint dies, and each is perfectly normal on its own. The error is the pairing. Because it takes the wrong die in the press and the mistake surviving long enough for coins to leave the Mint, few U.S. mules have left the Mint at all — the most famous modern one is known in fewer than twenty examples. Another is known in exactly one.
The word needs a moment's care, because it is used three ways and only one of them is a mint error:
- A mint-error mule — what this page is about. Dies of two different coins, paired by the Mint.
- A token or medal mule. In exonumia, "mule" describes any piece struck from dies not originally made to go together. Private die-sinkers did this routinely, and often on purpose — restrikes, fantasies, and tokens built from whatever dies were to hand. Many token and medal listings in this catalog carry the word in that sense. It describes the pairing; it does not mean anything went wrong.
- Mule Die Clash — when a mismatched pair of dies strike each other without a planchet between them, the clash marks they leave are a mule die clash. That is a die clash, recorded on the dies; the coins it produces need not be mules at all.
How Does It Happen?
A coining press holds an obverse die, a reverse die and a collar. Each die is made for one coin, and in normal production the right three meet. A mule happens when one of them belongs to something else.
Usually that is a mistake at installation: two dies of similar size and style sit in the same facility, and the wrong one is fitted. The size has to be close enough to work. The Philippine mules below were possible because the twenty centavos had been reduced in 1907 from 23 to about 20 millimetres, close to the five centavos' 21.3 — near enough that one coin's die could strike the other's planchet. Where the sizes differ more, the evidence stays on the coin: a smaller die strikes a design that does not reach the edge of a larger blank.
It is not always a mistake. In 1928 the Manila Mint had no reverse dies for the twenty centavos, so it struck the year's issue with a five centavos reverse instead — deliberately, as an expedient. Every one of those coins is a mule. The term describes what was paired, not whether anyone intended it.
A mule is a different thing from two errors it is often confused with:
| What was wrong | Example | |
|---|---|---|
| Mule | The dies belong to two different coins | Washington quarter obverse with a Sacagawea dollar reverse |
| Transitional Design | The dies belong to the same coin, from adjacent years or designs | A 1988 cent with the reverse introduced in 1989 |
| Wrong Planchet | The dies are right; the blank is not | A cent struck on a dime planchet |
How to Identify a Mule
- Read both sides as one coin. The denomination is spelled out on most U.S. coins — QUARTER DOLLAR on one face and ONE DOLLAR on the other is a mule, and nothing else explains it. Check the date, the denomination, and the design series against each other.
- Match the planchet to one of the dies. A mule is struck on the blank for one of its two dies — usually the one the press was set up for. Weight, diameter, colour and edge should all agree with that coin.
- Look for a design that does not fill the coin. When the stray die is smaller than the blank, its design sits inside a band of extra metal, often with a second rim. On a coin that should have a design reaching its edge, that band is a strong sign.
- For subtle mules, compare details rather than layouts. Where two dies are nearly identical in size and style, the difference is in the particulars. The 1918-S Philippine mule is recognised by its shield, which is wider than on the normal five centavos.
- Treat a claimed new mule as extraordinary. The populations of the famous ones are recorded closely. A coin claimed to be another example needs certification, not enthusiasm.
Notable Examples
2000-P Sacagawea Dollar / Washington Quarter Mule
The best-known mule of the modern era: a Washington state quarter obverse paired with a Sacagawea dollar reverse, struck on a golden dollar planchet at Philadelphia. The front shows George Washington with QUARTER DOLLAR beneath; the back is the Sacagawea dollar's soaring eagle. Because the quarter die is smaller than the dollar blank, Washington's portrait sits inside a band of extra metal — a second rim around a design that does not reach the edge.
It is often described the other way round, with Sacagawea on the front. This coin has George Washington on the obverse — the one with Sacagawea on the front is a different coin, known in a single example, below.
Frank Wallis of Arkansas found the first in May 2000, and the coin is often called one of the first genuine mule errors the U.S. Mint released, unlike the intentional mules struck in the 1800s. The Mint attributed it to a Washington quarter obverse die installed in place of a cracked Sacagawea obverse die. Three die pairs have been identified, and nineteen examples were publicly known as of 2019.
2000-D Sacagawea Dollar / South Carolina Quarter Mule
The reverse pairing, and unique: a Sacagawea dollar obverse struck with a South Carolina state quarter reverse, from Denver. PCGS authenticated it in 2022 and graded it MS64, and Coin World reported it as the only known example of this pairing.
Its existence explains a good deal of confusion. Accounts that put Sacagawea on the front of "the 2000 mule" are usually describing the Philadelphia coin wrongly, but a mule with Sacagawea on the front does exist — there is just one of it, and it is a different coin.
1918-S Philippine Five Centavos (Allen-4.08b)
Struck at San Francisco with a reverse die made for the twenty centavos — possible because the two coins had become almost the same size. The two reverses are very similar, which is why the mule is easy to miss: the diagnostic is the shield, wider than on the normal five centavos. Its reverse also carries a smaller date than the normal five centavos. It is catalogued as Allen-4.08b and FS-901.
1928-M Philippine Twenty Centavos (Allen-11.18)
The deliberate mule. With no twenty centavos reverse dies on hand, the Manila Mint used a five centavos reverse, and the whole issue — a reported 100,000 coins — is described as coming from that single die pair. Unlike most mules it is not a stray among normal coins: every 1928-M twenty centavos is one. The reverse shows the five centavos' narrower shield and its larger date. Catalogued as Allen-11.18 and FS-901.
Collecting Tips
- Certification is not optional. The famous mules attract fabrication. A two-faced coin assembled from two genuine ones is an altered coin, not a mule, and the difference is settled by examination, not by the photograph.
- Know the population before you believe a find. When a coin's known examples can be counted on two hands, a new one is news. Check it against the recorded census.
- Read the word in context. On a token, a medal or a colonial piece, "mule" is an ordinary description of the dies. On a regular-issue U.S. coin it is a die installation error with a short, well-recorded list of examples.
- A common mule is still a mule. Because the whole 1928-M twenty centavos issue is mules, it is the most readily found example of the pairing. It is a genuine example of the error.