Learn Grading: What Is a Mule?

Posted on 3/17/2020

These quirky notes are a fascinating area of numismatics, and some can bring a hefty premium.

PMG is proud to introduce Learn Grading, a new column that will help illuminate some of the terminology that is used in the paper money hobby. This distinct vocabulary often appears on PMG certification labels to call attention to properties of a banknote that are important to collectors.

This month’s topic is a numismatic quirk called a “mule.” In paper money collecting, a “mule” gets created when an overlap occurs with the plates used in the printing process. More specifically, the back plate that was used with one group of signers or series was also used with a different series or group of signers. This typically occurs when only one of the two plates is changed.

The resulting combination still works as a banknote, but sharp-eyed numismatists will notice the mismatching plates. It differs from a double denomination error, where plates from different denominations are mistakenly paired for the front and back.

On US notes, the ‘m’ stands for mule when it appears near the end of the Friedberg catalog number (Fr#) — from Paper Money of the United States, by Arthur and Ira Friedberg — though some Friedberg numbers consist exclusively of mules and omit the ‘m’ from the number.

One example of a mule is this 1934A Hawaii – World War II Emergency Issue $20 (Fr# 2305m). Graded PMG 67 Superb Gem Uncirculated EPQ, it realized $40,800 at a Heritage Auctions sale in January 2020, setting a record for a paper money mule in a Heritage sale.

1934A $20 - Hawaii World War II Emergency Issue graded PMG 67 Superb Gem Uncirculated EPQ.
Click image to enlarge.

In this case, it is the difference in the size of the print on the plate numbers that identifies it as a mule. The mule, pictured here with a non-mule counterpart, has what is known as “macro” numbers (1 mm tall) on the front, visible at the right side of the note partially obscured by the word HAWAII.

However, on the back, the mule has a “micro” number (0.6 mm tall) at the bottom-right (310), compared to its non-mule counterpart (384), which has the macro numbers that pair correctly with the front of the note.

From left: The mule and non-mule front, and mule and non-mule back of a 1934A $20 - Hawaii World War II Emergency Issue. The third image shows the “micro” plate number that identifies the mule. The other images all show “macro” plate numbers.
Click image to enlarge.

These Hawaii notes have an interesting background. Various 1934 and 1934A Federal Reserve Notes were printed with the word HAWAII at the left and right edges of the front of the note as well as across the entire back of the note. After the attack on Pearl Harbor in 1941 brought the US into World War II, there was concern that the Japanese might invade and occupy the island chain. This would give them access to the large amount of US currency used by residents there.

The solution was the Hawaii overprint notes, which could all be declared invalid if a large number of them fell into enemy hands. They were used in Hawaii starting in 1942. After World War II, nearly all of these notes were decommissioned by burning.

Mules for these Hawaii notes (with or without the asterisk in the Friedberg number to indicate a star replacement) have been identified in Fr# 2301 ($5), Fr# 2304 ($20) and Fr# 2305 ($20). Interestingly, the mules of Fr. #2301m, #2301m*, #2304m, #2304m* outnumber their non-mule counterparts in the PMG Population Report.

Similar World War II Emergency Issue banknotes were made for US forces in North Africa, with distinctive features that would allow them to be demonetized as a whole, if a large number were captured.

The Stack’s Bowers’ offering of the fantastic D. Brent Pogue Collection on March 19, 2020, contains an example of one of these notes (which had a pre-auction estimate of $20,000 to $30,000). The 1934 $10 North Africa – World War II Emergency Issue is graded PMG 66 Gem Uncirculated.

1934 $10 - North Africa World War II Emergency Issue pedigreed to the D. Brent Pogue Collection and graded PMG 66 Gem Uncirculated EPQ.
Click image to enlarge.

Like the Hawaii mule, this North Africa mule has an incorrect pairing of a macro number with a micro number. However, in contrast to the Hawaii mule, this mule has the “micro” number on the front (at the bottom right).

It is important to recognize that mules vary widely in scarcity. Also, it is not always the size of the plate number (as in these two examples) that determines a mule. The Pogue Collection includes seven other mules, including:

PMG attributes mules on US currency automatically and for no additional fee as part of the grading process.

Learn more

If you’d like to see more articles about paper money, sign up for free to become a member of the PMG Collector’s Society and then choose the PMG and related e-newsletters to be delivered to your inbox.

Stay Informed

Want news like this delivered to your inbox once a month? Subscribe to the free PMG eNewsletter today!


You've been subscribed to the PMG eNewsletter.

Unable to subscribe to our eNewsletter. Please try again later.

Articles List