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What is the correct way to pluralize an acronym?

My company deals in Purchase Orders.

For years they have referred to multiple purchase orders as POs. It seems, the proper spelling would be POes, because a P-O is referred to as a noun by everyone in the company, even though it is an abbreviated form of Purchase Order.

Which is correct? POs or POes ?

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Because PO is an abbreviation, it just gets an -s: POs. It's not the same as potato or tomato, which would get -es.

See Oxford: "Just add -s (or -es, if the noun in question forms its plural with -es). For example: MPs ... An apostrophe should never be used to form the plural of ordinary nouns, names, abbreviations, or numerical dates."

So: POs because order takes a simple s. And not PO's with an apostrophe.

However, I believe the Chicago Manual of Style does advocate using an apostrophe (but I don't have a reference for that, if it's online at all).

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  • So if the abbreviation was, say, GT for green tomato, are you saying the plural should be GTes? Or black ox becomes BOen? Looks wrong to me. Do you have any real-life examples where -es (or other non-s) us added?
    – Hugo
    May 9, 2012 at 6:54
  • See Alex B.'s answer for a CMOS reference: apostrophe only used for lowercase letters such as x's.
    – Hugo
    May 9, 2012 at 7:03
  • @Hugo: I wondered about quoting the whole thing. I can't say I agree with their reckoning on -es, and I find it difficult to think of real-world examples. Even "Co" for "Company" would look awkward as "Coes". Perhaps what Oxford published there is now anachronistic. There are other conventions too: in Canon Law, if you abbreviate Canon as Can then an acceptable plural is Cann, and that method can be used with others as well.
    – Andrew Leach ♦
    May 9, 2012 at 8:00
  • Reading the Oxford page, it's a page about when to add apostrophes, not specifically about how to pluralise abbreviations. I don't think they're suggesting at all that plural abbreviations should ever have -es. They're saying add -s (without apostrophe) to make a plural noun, or -es (without apostrophe) if the noun is made plural with -es.
    – Hugo
    May 9, 2012 at 9:44
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A Comprehensive Grammar of the English Language (1985) clearly states that abbreviations (understood broadly) get an -s (or 's, which was much commoner in the past). Section 5.81 (V)

The Chicago Manual of Style (16 th. ed.) says that "abbreviations usually form the plural by adding s. To aid comprehension, lowercase letters form the plural with an apostrophe and an s." (Section 7.14)

UFOs, CEOs, CFOs, CBOs, IRAs, URLs, BSs, MAs, PhDs etc. but x's and y's

The only exception I'm aware of is OSes.

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    Do you have a reference to show OSes is an exception? An answer (linked in the question comments) says OSs is most common
    – Hugo
    May 9, 2012 at 7:00
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It is POs, or more correctly P.O.s, since the plural is for the abbreviated word rather than the word "PO".

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    Can you please give a reference?
    – zpletan
    May 8, 2012 at 20:24
  • There are only a few instances where "...os" is wrong and "...oes" must be used, and those words are already established. ESL @ About.com. Since "PO" (in this question) is not a word and has not been established to be one that must use the "es" it would use a single "s".
    – BillyNair
    May 8, 2012 at 20:35
  • I agree it is POs, but adding periods to abbreviations is somewhat old-fashioned in these acronym-heavy times.
    – Hugo
    May 9, 2012 at 6:57

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