Plurals of acronyms, abbreviations, initialisms and single letters do not usually take apostrophes. Guidance on this issue is not unanimous, but some general advice can be consistently applied, and may help resolve some of the widespread confusion that seems to be generating even more widespread confusion.
Pictured is a screenshot from Twitter, taken a few months ago, showing popular subjects or “Trending Topics”. The vast numbers of people using Twitter means that for a subject to become a “Trending Topic”, it needs to be mentioned a great deal. Because the software distinguishes between two words that are identical apart from the presence or absence of an apostrophe, both “SATs” and “SAT’s” have the potential to appear concurrently in the list, as indeed they did. Evidently, both forms are widely used. My preference is for “SATs”, and I would consider “SAT’s” ill-advised, because such apostrophes are largely unnecessary and potentially confusing.
A recent report from TechCrunch illustrates the potential for miscues from apostrophised plurals:
Readers could easily begin reading the sentence “The URL’s already shortened…” and think that “URL’s” is a contraction of “URL is (or has)”: “The URL is (or has) already shortened…” These miscues would cause only minor irritation, but why invite them at all? The problem is easily and sensibly avoided by adopting the widespread convention of adding ‘s’ to form a plural. This is what Nambu Networks, the company TechCrunch are reporting on, have done (see the bottom line in the screenshot). Unnecessarily apostrophised plurals, as well as being unsightly, almost certainly increase general confusion about plural formation. Examples are endless.
Grammar is a minority interest, so it is unsurprising that this usage has become common. So many people have seen signs like the abovelinked, or ads for “CD’s” and “DVD’s”, that they reasonably assume this to be the standard or “correct” form. As a convention it is well on the wane. Most modern advice recommends including an apostrophe only if there is a danger of misunderstanding or ambiguity. So we have ABCs, ATMs, CDs, CD-ROMs, CEOs, CPUs, DTs, DVDs, EEGs, GDPs, GCSEs, GPs, IOUs, IQs, MAs, MPs, NGOs, PCs, PhDs, SATs, SMEs, SOSs, TVs, UFOs, URLs, VCRs, VIPs, WMDs, and so on. Inserting apostrophes in some, e.g. ABC’s, is not an offence to legibility, but I would consider it dubious and unnecessary. That goes for initialisms ending in ‘S’, too.
“Sellers of CDSs [credit-default swaps] had no such record to consult.” (The Economist, 8 Nov. 2008)
“Modern EEGs work on the same principle as the sprung mirror galvanometer” (Empson, Sleep and Dreaming)
“Treasury bills (TBs) are short-term UK government IOUs of three months’ duration” (Blake, Financial Market Analysis)
“Finally, we must consider what class of NLPs can be treated by PWL approximation using SOSs” (Hartley, Linear and Nonlinear Programming)
Incidentally, I searched the British National Corpus for examples of “WMDs”, but I didn’t find any:
Much as I love the Corpus, I never expected it to deliver such a hearty Monday laugh.
Some abbreviations are also commonly or occasionally written with full stops, for example academic qualifications and country names. When these are pluralised, -s or -‘s can be added: D.T.s or D.T.’s, G.I.s or G.I.’s, Ph.D.s or Ph.D.’s. There’s a fair argument that an apostrophe here increases clarity. Some abbreviations already stand for plurals, e.g. U.S. (or US) for United States. So there is no need for USs, U.S.s or U.S.’s, except the latter as a possessive. Abbreviations of length, volume, weight and time are usually left unchanged, i.e. without appending either -s or -‘s, e.g. ft., min., sec., lb., oz. There are a few common exceptions: yrs., hrs. Many such abbreviations are often written without the full stop: mm, cm, km, oz, lb. This is fine.
The apostrophised form is also accepted with pluralised single letters. In fact, it is sometimes recommended to avoid confusion: Mind your p’s and q’s, dot your i’s and cross your t’s. This is not grammatical, but it is typographically justifiable. Italics are also okay: Mind your ps and qs, dot your is and cross your ts – but this presentation can be too subtle for some eyes, fonts, and publishing houses. Upper case letters are clearer: Mind your Ps and Qs. But this increase in clarity can be negligible: Dot your Is. So the preferred option for this last example would be to include an apostrophe, especially with lower case i’s, or to wrap the letter in quotation marks: Dot your “i”s and cross your “t”s. In essence: with single letter plurals, if there is the possibility of ambiguity or confusion, apostrophes are preferred but not necessarily mandatory.
She got four A’s in her GCSEs
“Mississippi” has four i’s and four s’s
They decided to play X’s and O’s (or “Xs and Os”)
We were taught the three R’s (or “three Rs”)
A final point: like pluralised abbreviations, decades and centuries written as numerals once commonly ended in apostrophe-s: 1800’s, 1960’s. Nowadays, however, the apostrophe tends to be left out: 1800s, 1960s. The same goes for abbreviated decades: ‘60s, not 60’s or ‘60’s. This trend pleases me.