According to the Chicago of Manual of Style, the apostrophe has three primary uses: to show possession, to indicate missing letters (i.e. in contractions), and, “…rarely, to form the plural of certain expressions.” Perhaps the most egregious apostrophe error is its overuse.
An apostrophe is not used in the following situations:
- to form the plural of a family name. We went along with the Smiths to the beach.
- when capital letters are used as words, numerals are used as nouns, and abbreviations are plural e.g. Capital Bank’s IRAs are earning a higher interest rate than First Federal’s. (No apostrophe in IRA – only an s to form the plural.) Likewise: The hits from the 1990s continue to attract listeners. (No apostrophe in 1990s.) Chicago makes an exception in the case of the plural of lowercase single letters, but for little else. They allow apostrophes in Mind your p’s and q’s – and that only for the aesthetics. You have to admit that even in context ps and qs is a challenge to decipher. But dos and don’ts do not rate apostrophes.
- in possessive pronouns: hers, his, theirs, its. (see my previous post about the appropriate use of the apostrophes in it’s.
The following situations require apostrophes:
- The possessive form of most singular (regular) nouns is formed by adding an apostrophe + s: horse’s mouth; chair’s leg; Jamie’s daughter. This formulation is a less cumbersome way to show possession than the prepositional phrase such as the mouth belonging to the horse.
- The possessive of most plural nouns is formed by adding an apostrophe after the s: girls’ toys, books’ pages, clocks’ batteries. However, in the case of nouns that do not add an s to form their plural, e.g. man – men, woman – women, child-children – the possessive is formed by apostrophe + s: man’s – men’s, woman’s – women’s, etc. This the same way singular nouns form the possessive.
- After waffling on whether to add only an apostrophe or apostrophe + s to nouns that end in s, the 16th (latest) edition of CMOS comes down on the side of apostrophe + s, thus Des Moines’s mayor attended the conference; Descartes’s thesis was included in the syllabus are the proper renderings.
- An apostrophe is properly used to indicate missing letters. Common contractions are the combination of a pronoun and verb such as we are – we’re; he/she is – he’s/she’s; they have – they’ve and be-verbs and most of the auxiliary verbs when followed by not: are not – aren’t; was not – wasn’t; have not – haven’t.
And as for that rare use to “form the plural of certain expressions” the writer is advised to simply add an apostrophe in phrases such as these: for goodness’ sake and for righteousness’ sake.