As generally agreed and as extensively discussed in this question, "an" should be used in place of the more common "a" where the following word begins with a vowel sound.

I have just encountered for the first time an instance of the phrasing

an mule

in the book Why Chemical Reactions Happen (James Keeler and Peter Wothers, Oxford University Press, 2003). (This book is not mine. I was sitting opposite a friend when the friend mentioned the unusual phrasing.)

This phrasing appears to contradict the rule discussed above, since "mule" is, as far as I am aware, pronounced /mjuːl/ (see Wiktionary).

Here is the context. The author is using an analogy to explain a chemistry concept (the italics are not mine, but the bold emphasis is):

A useful analogy here is that of an mule, which we get by crossing a horse with a donkey. To describe an mule as a horse or a donkey would be inaccurate and it certainly does not interconvert rapidly between the two! Rather we need to recognize that although an mule has parts that are reminiscent of a horse and parts that a reminiscent of a donkey, it is something altogether different.

Looking for other examples online, I have managed to find one on Wikipedia's List of Siege engines. Discussing a Siege engine, the Scorpio:

Similar to the ballista, but smaller. Was sometimes mounted on an mule-drawn cart.

Other examples are difficult to find, but here's one from a user-posted question on Answers.com (referring, notably, to a kind of deer rather than the animal, a mule):

Is an mule deer an omnivore?

No. Mule deer are herbivores. ...

What is going on? Is there an irregular pronunication of "mule" of which I am not aware? Is this some strange language quirk, some archaic spelling that I am encountering for the first time?

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2 Answers 2


The original printing had "an ass" instead of "an mule". That error was not lost on the chemistry community. See, for instance, Krenos's 2004 review of the text in the Journal of Chemical Education:

In Chapter 10, bonding in extended conjugated systems and resonance are introduced with the horse + donkey = ass analogy (it is likely the hybrid animal mule is intended instead of ass, however).

It seems the editors subsequently made an error mistake.

  • 5
    I am marking this answer as correct for providing strong evidence for the source of the typo.
    – C Ren
    Commented Sep 11, 2019 at 8:44
  • 45
    +1 for the "an mistake" :)
    – Tvde1
    Commented Sep 11, 2019 at 13:24
  • 11
    Reminds me of the urban legend about an author who, at the last minute, decided to change a character's name from David to Jeff. After it had gone to print, and much too late, he remembered that there was a scene that took place in the Victoria and Albert Museum involving the replica statue, Michelangelo's Jeff. Commented Sep 11, 2019 at 14:32
  • 6
    This has also happened in the opposite direction: an umpire was originally a noumper (non-peer) and a newt was once an ewt or an eft.
    – Davislor
    Commented Sep 11, 2019 at 19:24
  • 1
    Perfect explanation. :) Commented Sep 11, 2019 at 22:43

It's just a typo, and it probably originally said "an ass". That would have been changed because it's incorrect (an ass is a donkey, not a horse–donkey cross) and, even if it were correct, the fact that ass means well, you know, ass, might be distracting enough to make it worth changing.

Another source of this kind of typo is when an adjective has been added or removed and the article hasn't been updated (e.g., "an elephant" becoming "an big elephant" or "an amazing coincidence" becoming "an coincidence"). That's presumably not happened in this particular text, since there are several instances of "an mule".

  • 52
    @DavidM Apparently, yes. If they'd posted it as an answer, I'd have seen it; this is part of the reason why we don't post answers in comments... Commented Sep 9, 2019 at 21:17
  • 12
    Actually, ass is rather different from arse.
    – Andrew Leach
    Commented Sep 9, 2019 at 22:02
  • 9
    "since there are several instances of "an mule" ... or somebody used find all and replace.
    – muru
    Commented Sep 10, 2019 at 1:42
  • 12
    What about the mule deer example? Just a typo on it's own? I can't imagine an ass deer is a thing (and I'll admit, I'm afraid to Google it). Commented Sep 10, 2019 at 2:22
  • 23
    @AndrewLeach: In traditional British English, sure, they’re different: the farm animal is an ass, your buttocks are an arse — they’re spelled differently, and for many speakers they’re pronounced differently too (depending on accent). But in American English, both the buttocks and the animal are an ass, and are pronounced identically — perfect homonyms. (And using ass for the buttocks is now becoming more common in British English too, both in spelling and pronunciation.)
    – PLL
    Commented Sep 10, 2019 at 12:59

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