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For example, if I wanted to write the equivalent of

There are many automated teller machines in this city.

Would it be

There are many ATMs in this city.

or

There are many ATM's in this city.
(could get confused with possessive form or contraction).

or just

There are many ATM in this city.
(assuming the final s is included in Machines represented by M).

Maybe something else?

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9  
+1; great question, but whilst not meaning to sound overly pedantic, ATM is an initialism, not an acronym. –  Bryan Apr 14 '11 at 22:22
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ATM: "Noun. ATM (plural ATMs). 1. (banking) Initialism of automated teller machine." –  Peter Mortensen Sep 16 '11 at 18:11
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@Bryan, not all dictionaries agree on that - some define acronyms to include initialisms and your pedantry fails to mention that... :) –  Unreason Oct 11 '11 at 12:28
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9 Answers

up vote 61 down vote accepted

The Chicago Manual of Style has an interesting way to address this: They omit the apostrophe, unless there are periods in the abbreviation. So this would give you ATMs, or alternately A.T.M.'s. (A.T.M.s looks weird.) chicagomanualofstyle.org, "Plurals"

This page indicates that acronyms ending in the letter "S" get an apostrophe, something I've seen before, but can't find in a general reference. So one would write ATMs and SOS's.

This page on the North Carolina State University website references AP's rule as being to always use an apostrophe.

The 2009 AP Stylebook's "plurals" entry has no section on acronyms, but mentions "VIPs", I can't find anything addressing how to specifically pluralize acronyms. (The "abbreviations and acronyms" section is also of no help.)

Personally, I omit using apostrophes unless I can't avoid it. I do use them when talking about single letters or where it would avoid confusion. (For example, SOs for "Significant Others" looks like an incorrectly capitalized SOS.)

To paraphrase Carol Fisher Saller, the clearer usage is the correct one.

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11  
'VIPs' is a strange one. It could be argued that the expanded acronym, when pluralized, should be 'Very Important People' rather than 'Very Important Persons'. Using this argument, 'VIP' could be used as a singular or plural noun. "The VIP has arrived." "The VIP have arrived." I'm not sure anyone else would agree with my logic, though. –  oosterwal Mar 7 '11 at 18:24
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Acronyms ending in the letter “S” take -es in the plural: “Your SOSes are getting through to no one.” –  tchrist Apr 1 '12 at 1:38
    
@tchrist - Source? –  Neil Fein Apr 1 '12 at 2:10
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The crux of the problem is that SOS’s cannot serve as both a genitive singular and a nominative plural, because you then run into a brick wall trying to make a genitive plural: *SOS’s’s or some such similar silliness simply doesn’t work. “These SOSes are new, this SOS’s origin, these SOSes’ origins,” etc. are all clear and unambiguous. You can’t do that with SOS’s trying to do both jobs and getting klutzed up when it gets promoted to a genitive plural. –  tchrist Apr 1 '12 at 3:03
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A.T.M.'s looks weirder than A.T.M.s to me; of course, A.T.M. looks weird in the first place. Also, as @tchrist said, I have generally seen SOSes, etc. –  Tortoise Nov 6 '12 at 1:30
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The first is the correct usage, in my view. The third may be quite acceptable however, since the M in ATM could equally stand for 'machine' or 'machines', though I think pluralising the actual acronym is much clearer in speech.

In any case, never use an apostrophe. 's should only be appended to a word to create the posessive form ("of ..."), never for plurals.

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Unfortunately I think ’s is the standard way to pluralize single letters—“A’s, B’s, and C’s”. –  nohat Aug 12 '10 at 21:27
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@nohat: Possibly, though I'm not sure of that myself. Either way, letters aren't technically acronyms, so I think my answer is still safe. –  Noldorin Aug 12 '10 at 21:29
    
@nohat idioms.thefreedictionary.com/_/… –  Juan Mendes Mar 15 at 8:49
    
@JuanMendes: that's bad usage in my view. It's done often enough though. –  Noldorin Mar 15 at 18:42
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I vote for the first, "ATMs". The second is just wrong (apostrophe is not used for plurals, ever). This is because ATM is a defined term for an AT machine, and using it as plural "automatic teller machines" would be a redefinition of a common abbreviation, which one should not be trigger-happy about.

The third, however, does not solve the real need to say there's more than one. It is though clear from the sentence, but might not be so in a general case.

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I disagree that the third is the most correct. Either of the first two is much more useful than the third—as you say, as well as much more widely used. –  jeffamaphone Aug 12 '10 at 21:24
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Not really looking for the most popular way. I was more interested in some authoritative rule on the correct way. –  JohnFx Aug 13 '10 at 15:20
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Depends how you see it JohnFx - for me the most popular way is the correct way - usage makes the rules. There is no "authority" who can decide what is right or wrong. –  Evan Aug 13 '10 at 19:10
    
thanks, edited. –  Pavel Radzivilovsky Aug 14 '10 at 21:50
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Actually, an apostrophe is used for plurals in at least one special situation: single letters. As in mind your p’s and q’s. If you think about it, it has to work that way: you don’t dot your *is; you dot your i’s, and necessarily so. –  tchrist Feb 24 '12 at 1:12
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Using 's to pluralize something is called a "Greengrocer's apostrophe".

I think the battle against the Greengrocer's apostrophe is one we're bound to lose - even if grammar of the general population improved, we'd still occasionally be facing nouns which have a mixture of upper and lower case, for which adding an s by itself at the end would be confusing.

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What about greengrocers whose name has an S at the end of the surname, like Ralphs, a la George A. Ralphs? –  Jared Updike Aug 11 '11 at 18:26
    
@Jared - You pick a method and stick to it. –  Neil Fein Feb 26 '12 at 21:59
    
Since most readers will recognize easily what is meant when a combination of characters that don't form a "normal word" is followed by an apostrophe and a suffix (typically 's'), why should one "battle against" such uses in contexts where omitting the apostrophe might potentially cause ambiguity? Some people claim uppercase letters don't need the apostrophe. As As, Is, Os, and Us form words when Ss are appended, I think that notion is silly. –  supercat Oct 18 '12 at 23:09
    
"Greengrocer's apostrophe" is a term for incorrect use of 's. It does not apply to uses of 's that are merely much less common than they once were. –  Jon Hanna Feb 13 '13 at 21:41
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Either of the first two is acceptable, but I would recommend the first as the apostrophe isn't needed to convey your meaning, and as such is not required.

The third is just wrong since it creates an awkward sentence that is hard to say and discomforting to read. Most acronyms, including ATM, have a well-defined and commonly accepted meaning, which very rarely includes the pluralization. Avoid the ambiguity and include the s.

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Oxford Dictionary [e.g. SOS, noun (plural SOSs)] and The Economist [e.g. Are ATMs stealing jobs?] both go for the first option.

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[citation needed] –  Sridhar Ratnakumar Aug 12 '10 at 21:05
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I agree with Wikipedia, wordreference and CMOS - acronyms and initialisms are "regular" nouns; plurals are formed by adding "s".

Checking Google Books for actual usage in a relatively "contentious" case, I searched for:

"OSs" unix windows linux 3120 written instances

"OSes" unix windows linux 1060 instances

"OS's" unix windows linux 520 instances

"Simpler" cases such as CDs vs CD's are even more decisive (over 10:1 in favour of the former).

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There is a problem with product names. For example, Nikon has models named (each in the singular) D3, D3s, D3x. Trying to make plurals and possessives and plural possessives of those is a real treat. How many D3s do you have? None, we only have a D3s, not a D3. How many D3ses do you have? How many D3xes do you have? –  tchrist Feb 24 '12 at 1:15
    
@tchrist: Yawn. I might have known you'd come up with a gotcha!. Nikon might also make a model D3es for all I know. Basically that's their problem. If we decide to endorse D3es's they'll probably release a special model with a name ending in apostrophe+"s". –  FumbleFingers Feb 24 '12 at 1:27
    
I would be very skeptical of any searches that include apostrophes. –  MrHen Feb 24 '12 at 5:29
    
@MrHen: You need to constantly re-evaluate your thinking. It's pretty obvious the backroom boys at GB are constantly updating their software, and it won't have escaped their notice that we keep including apostrophes in our search terms. I looked at that resultset, and it seems to me they are now pretty accurate at detecting, indexing, and retrieving apostrophes - esp. on recent text such as would include CD's. –  FumbleFingers Feb 24 '12 at 6:22
    
@FumbleFingers: The issue is not only with GB. Every step of the process can choke on an apostrophe. Simply looking at the result set is not going to provide you with enough information to tell you whether it is accurate. As such, I would be very skeptical of any searches that include apostrophes. –  MrHen Feb 24 '12 at 23:42
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Using the apostrophe to indicate plurals of numbers, letters and abbreviations is standard, but it is not as common as s without the apostrophe.

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I'd say you have it backwards. Most authorities (e.g., stylebooks) prefer "ATMs," but most ignor--. Sorry, most people tend to use "ATM's". –  Jon of All Trades Feb 24 '12 at 2:01
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Since this is a question about acronyms, and the Federal Government's bureaucracy is notorious for using acronyms, I decided to look up the answer in the United States Government Printing Office (GPO) Style Manual (2000).

Rule 8.11 of the GPO Style Manual states: "While an apostrophe is used to indicate possession and contractions, it is not generally necessary to use an apostrophe to show the plural form of most acronyms, initialisms, or abbreviations, except where clarity and sense demand such inclusion." As examples, the rule suggests: OKs ABCs RIFs YWCAs

The rule does not show an exception for an acronym, but does refer to one case I found interesting -- the "Oakland A's" needs an apostrophe because otherwise it would be the "Oakland As." From that I would assume that if the addition of an s to an acronym would appear to give the acronym a different meaning, then an apostrophe would be in order. But since acronyms are capitalized letters, the addition of a small s should not make a difference, except where (for some reason) one is writing in a format that is all capitals -- such as the format that military and diplomatic messages were sent until very recently.

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Single letters do seem to take apostrophes idioms.thefreedictionary.com/_/… –  Juan Mendes Mar 15 at 8:51
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protected by Jasper Loy Jun 12 '12 at 20:00

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