What is the correct way to pluralize an acronym / initialism?

Solution 1:

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.

Solution 2:

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).

Solution 3:

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.

Solution 4:

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

Solution 5:

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.