Almost everybody knows that it is wrong to use apostrophes before an ‘s’ which denotes a plural rather than a possessive. So it is “breakfasts” and “tomatoes”, not “breakfast’s” (which I saw on a sign yesterday) or “tomato’s”.
But what about plurals of words made up of initial letters, like “GCSE” or “CEO”? (Incidentally, these should – strictly speaking – be referred to as initialisms, rather than acronyms – see explanation below)*. Does the same rule still apply?
Things get a little more complicated when it comes to plurals of single letters. Here the authorities do not agree. The Times style guide advises that “an apostrophe should be used to indicate the plural of single letters - p's and q's”**. This is presumably to avoid confusion with two-letter words (or initialisms) ending in "s", like "ps", "as" or "is",
* An acronym is made up of initials, or first syllables, of other words, and is pronounced as if it were a word – like NATO (North Atlantic Treaty Organisation) or Unicef (United Nations International Children’s Emergency Fund). With an initialism like CD, MP, GCSE or CEO, each of the initials is pronounced separately.
** The etymology of this phrase, which means “to be careful or particular in one's words or behaviour; to mind one's manners”, is mysterious; The Phrase Finder discusses it at some length. Some people think it stems from advice to apprentice typesetters or children learning to write (not muddling up two similar-looking letters), but it has also been suggested that it comes from “pints and quarts” or “pleases and thank-yous”.