When I was in high school back in the 1970s, I was taught that to make a plural of an acronym, a letter, or a number, one should add an apostrophe and "s". Like I would have written this sentence, "... back in the 1970's ..." I would write "one CD, two CD's". Etc. I followed this rule faithfully for years until a co-worker told me it was wrong. Now I can't find any source that agrees with what I was taught.

Is this a rule that has changed over time? Was the convention in the 70s that one should use an apostrophe but this has changed and now we do not? Or were my high school English teachers just confused?

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  • It is simple: For all letters capitalized or not and for numbers, both the use and the lack of an apostrophe is correct. See the OED.
    – user84593
    Commented Sep 11, 2014 at 10:03
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    By the way, you still need an apostrophe in 70s. At the beginning. To signify that you left out the 19. That is, '70s Commented Apr 10, 2015 at 9:42
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    I dunno what's the rule, but I'd burn plurals' apostrophes with fire. They're totally trashy and uneducated lookin'. Commented May 26, 2020 at 8:07
  • CDs weren't available to the general public until ~1983, so I doubt you'd have written "CD's" in the 1970s.
    – cosimo193
    Commented Jun 3, 2021 at 16:23

8 Answers 8


I seem to remember the old askoxford.com site said either was acceptable: CDs and CD's.

But now the replacement Lexico powered by Oxford firmly suggests to avoid the apostrophe except in a few special cases:

Apostrophes and plural forms

The general rule is that you should not use an apostrophe to form the plurals of nouns, abbreviations, or dates made up of numbers: just add -s (or -es, if the noun in question forms its plural with - es). For example:

MP MPs (e.g. Local MPs are divided on this issue.)

1990 1990s (e.g. The situation was different in the 1990s.)

It's very important to remember this grammatical rule.

There are one or two cases in which it is acceptable to use an apostrophe to form a plural, purely for the sake of clarity:

  • you can use an apostrophe to show the plurals of single letters:
    • I've dotted the i's and crossed the t's.
    • Find all the p's in appear.
  • you can use an apostrophe to show the plurals of single numbers:
    • Find all the number 7’s.

These are the only cases in which it is generally considered acceptable to use an apostrophe to form plurals: remember that an apostrophe should never be used to form the plural of ordinary nouns, names, abbreviations, or numerical dates.

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    Hmm, the wording of the first paragraph you quote implies that many people do (or at one time did) use an apostrophe. That could be evidence for conventions having changed or their being some disagreement about best practice. Note I'm not arguing for a different convention; I'm just wonderingl
    – Jay
    Commented Jan 26, 2012 at 7:20
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    I don't understand why "you can use an apostrophe to show the plurals of single numbers"; what's unclear about "7s"?
    – NiteCyper
    Commented Apr 22, 2014 at 17:18
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    @NiteCyper, indeed, "7's" is more unclear because "Seven" is used as the first name for some children of very creative parents (and not-so-creative fictional parents on TV). Though numerals are not legal names in the US for people, they are certainly "legal" for pets and in some foreign countries.
    – hobs
    Commented Aug 8, 2014 at 16:17
  • What about acronyms for plural words (such as Curricula Vitae, Very Important People or Relational Data Base), where the plural noun does not end in s or es? I've asked in another question but it's been closed with the insistence that this response answers that. Could you clarify?
    – Keith
    Commented Jan 25, 2017 at 12:04
  • They're commonly written as CVs, VIPs, RDBs. Acronyms often forms a new lexical unit following normal rules for plurals. Similarly lasers (from "light amplification by the stimulated emission of radiation") and radars (from "radio detection and ranging").
    – Hugo
    Commented Jan 25, 2017 at 20:36

The Chicago Manual of Style says:

Capital letters used as words, numerals used as nouns, and abbreviations usually form the plural by adding s. To aid comprehension, lowercase letters form the plural with an apostrophe and an s.

The lowercase letter exception presumably exists because omitting the apostrophe can make the sentence much harder to understand (for example, "mind your p's and q's" is much clearer than "mind your ps and qs"). I don't know if the rule was different in earlier editions of Chicago, but it's very much in keeping with the guide's philosophy of prioritizing clarity and simplicity over prescriptivist rules.

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    Obviously you're not a Unix user; mind your ps means something completely different
    – Foon
    Commented Aug 18, 2016 at 14:01
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    @Fattie - The p's and q's example may not be the best. What about a's and i's? Without the apostrophes, that sentence reads: "What about the as and is?" Clearly, the apostrophes add clarity. Commented Mar 31, 2017 at 3:27
  • Hey @aridlehoover, actually FWIW I think "the as and is" is totally clear. Indeed, working in typography and/or with computers, one says that all the time.
    – Fattie
    Commented Mar 31, 2017 at 10:57
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    I wouldn't exactly take "The Chicago Manual of Style" as an authoritative source, especially given that it contradicts the OED. Commented Mar 23, 2018 at 13:47

It doesn't matter how many different authorities/style guides are cited - usage in this area has never been fixed, so it doesn't mean much to suggest the "rule has changed over time".

The use of apostrophes has always been less common, but it's been around at least a century (there, for example, the 1700's). It's also worth noting that (particularly in earlier years) writers would sometimes simply leave a space (i.e. - write the 1700 s).

Per other answers, I suggest only using the apostrophe in contexts where not doing so would make things hard to read (as in dotting the i's and crossing the t's). That's in line with my understanding that the general trend over many decades has been a reduction in the use of all punctuation where the primary justification was to follow "rules of grammar". Increasingly, the primary (or only) justification for punctuation marks is improved legibility.

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    @Barrie While I agree, the problem with that is obviously that school does teach rules (which is in a way a shortcoming of the education system). So there observably are rules. Commented Jan 26, 2012 at 10:30
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    @KonradRudolph: It depends what you mean by a rule. Commented Jan 26, 2012 at 12:05
  • @Barrie What I meant to say is simply that teaching is inadequate. As this question clearly shows, pupils routinely get a wrong impression of how language actually works. Commented Jan 26, 2012 at 12:55
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    @KonradRudolph: Oh, yes, certainly. Most people don't have a clue about how language works. Commented Jan 26, 2012 at 13:25

My copy of Practical English Usage, 2nd Edition (Michael Swan, 1995) says this:

Apostrophes are used in the plurals of letters, and often of numbers and abbreviations.

  • He writes b's instead of d's.
  • It was in the early 1960's. (OR ... 1960s.)
  • I know two MP's personally. (OR ... MPs.)

Using Amazon's Search Inside The Book I can see that these recommendations haven't changed in the 3rd edition, published in 2005.

So there's at least one source that agrees with what you were taught!

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    Interesting that this disagrees with advice from other books by the same publisher, e.g. Oxford Guide to Style ("Do not use the apostrophe when creating plurals. This includes names, abbreviations (with or without fall points), numbers, and words not usually used as nouns...")
    – Jules
    Commented May 18, 2017 at 4:31
  • It is interesting that books by the same publisher, but different autors give conflicting advice. The solution to the conundrum is surely an indication that personal preference is being passed off as a rule.
    – Cato
    Commented Nov 9, 2017 at 16:21

There doesn't seem to be any consensus on this at the moment -- different style guides have different opinions:

New York Times:

Use apostrophes for plurals of abbreviations that have capital letters and periods: M.D.’s, C.P.A.’s. Also use apostrophes for plurals formed from single letters: He received A’s and B’s on his report card. Mind your p’s and q’s.

But do not use apostrophes for plurals of abbreviations without periods, or for plurals formed from figures: TVs, PCs, DVDs; 1990s, 747s, size 7s.

The AP:

Use apostrophes to form the plural of single letters but not figures or multiple letters.

I think I agree with the U.S. Government Printing Office:

While an apostrophe is used to indicate possession and contractions, it is not generally necessary to use an apostrophe simply to show the plural form of most acronyms, initialisms, or abbreviations, except where clarity and sense demand such inclusion.

[emphasis added]

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    The sources you cite do not seem to be "of different opinion" so much as they simply cover different cases. NYT is the same as AP except that dotted abbreviations take the apostrophe. (Dotted abbreviations look old-fashioned and are probably being phased out anyway; thus the omission from AP.) The USGPO doesn't cover numerals or "literal" letters at all (A's, B's, p's, and q's are not acronyms, initialisms, or abbreviations).
    – John Y
    Commented Jan 25, 2012 at 23:28
  • My point here is, 'size 7s' - would that be read as 'size sevens' or 'size seven ess'? Because 'size seven ess' seems highly plausible to me as a sizing code - without further context.
    – Cato
    Commented Feb 5, 2018 at 10:40

The question is has the rule changed?

Yes. For example, the Webster’s New Collegiate Dictionary, 1981 edition, gives this rule in the section “Handbook of Style”:

Apostrophe ’

  • often forms plurals of letters, figures, and words referred to as words
    ⟨You should dot your i’s and cross your t’s.⟩
    ⟨His l’s and his 7’s looked alike.⟩
    ⟨She has trouble pronouncing her the’s.⟩

No examples or guidance are given for omitting the apostrophe. This is different from the common advice today to use omit the apostrophe except when unavoidable.

  • This begs the question, assuming there is / has been a single rule. FF's 'It doesn't matter how many different authorities/style guides are cited - usage in this area has never been fixed, so it doesn't mean much to suggest the "rule has changed over time".' would seem give the true picture. Commented Oct 4, 2016 at 23:31

See Lynne Truss's 'Eats, Shoots and Leaves' (2003). She writes on p46:

only one significant task has been lifted from the apostrophe's workload in recent years: it no longer has to appear in the plurals of abbreviations ... . Until quite recently, it was customary to write "MP's" and "1980's" - and in fact this convention still applies in America.


Hmm. I recall having learned (in the '80s, in the U.S.) that letters, numbers, and words used as "themselves" (I would call these literals today, partly due to being a programmer) do take the apostrophe when forming the plural. Examples that mostly corroborate this, taken from a dead-tree American Heritage Dictionary, 2nd College Ed. (c) 1982, 1985:

42's and 53's

x's, y's, and z's

in the 1700's

an article with too many also's

I actually don't think I learned years that way (the rationale for leaving off the apostrophe for years would be that the digits in years are not functioning as figures per se, plus I'm sure I learned that '70s should have the leading apostrophe to mark the omission of digits but not the pluralizing apostrophe).

So it appears you have good support for using the apostrophe for single letters (everything we've seen so far) and some support for numbers (the old dictionary I cited here). This source doesn't say anything about pluralizing abbreviations of any kind, though.

Finally, I will chime in with everyone who recommends being sparing with apostrophes. I would really love to not feed the common tendency to pluralize randomly or overzealously with apostrophes. The less people see I bought 3 CD's, the less people will be inclined to write I bought 3 apple's. (I hope.)

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    Taking an example which will have very few "false positive" - Google Books written references to 78rpm records in the 50s, it's clear that on average people didn't follow your "rule" in that decade - they preferred 78s over 78's by a factor of about 2:1. Commented Jan 26, 2012 at 16:03
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    @FumbleFingers: My point is that I side with those who feel that the grocers' apostrophe should go away.
    – John Y
    Commented Jan 26, 2012 at 22:56
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    @JohnY: Perhaps I favor apostrophes more than most people because I frequently write about things like electronic components, whose part numbers often have weird suffixes but don't contain apostrophes. Even if "74HC595s" would arguably be no less ambiguous than "74HC595's" because no 74HC595 variants happen have a single "s" as a suffix, I can't say for certain that no manufacturer makes, or will ever make, a part called a "74HC595s". Using apostrophes only when strictly necessary for disambiguation seems less practical than using them consistently...
    – supercat
    Commented Oct 17, 2012 at 6:15
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    @supercat: I'm not arguing for inconsistent use of any punctuation, or for minimal use of any punctuation. English rules and guidelines are already inconsistent: we've got "his, hers, and its" which are possessives without apostrophes, "p's and q's" which are plurals with apostrophes, and all manner of exceptions to just about any rule you care to formulate. And on top of all that, English is fluid and evolving. Honestly, I think apostrophe use should be simpler and more consistent, but for the purpose of this question, I was just reporting what my teachers and dictionaries told me.
    – John Y
    Commented Oct 17, 2012 at 13:29
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    @JohnY: Fair enough, though I think that fair discussions should mention that previous decades' usage guides encouraged more liberal use of apostrophes than modern ones--in some ways curious since the number of situations which are ambiguous without the apostrophe has if anything increased over the years. It's too bad there's no other non-syllable-breaking equivalent to the hyphens in "re-cover" and "re-press" (which ensure the prefix remains identifiable as such). Writing &'s rather than &s may be seen as antiquated, like writing "coöperation", but it's certainly not unclear.
    – supercat
    Commented Oct 17, 2012 at 14:51

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