The rules about forming possessives can seem confusing, especially because forms differs according to which style manual a publication is following. But the primary styles are relatively simple.
In this post, I will refer only to the two prevalent styles in mainstream publishing: that of the Associated Press Stylebook, the resource of record for American newspaper publishing, as well as some magazines, many websites, and various other forms of written communication, and The Chicago Manual of Style, the bible of American book publishing, as well as many general-circulation periodicals. Guides for certain academic and scholarly subjects may differ.
Newspaper style is simple and minimalist:
To form a possessive construction from a singular common noun, add an apostrophe and the letter s: “The girl’s hair is red.” However, if the noun ends in the letter s and the following noun begins with an s, add an apostrophe only: “The waitress’ smile was infectious.”
Treat a proper noun the same as a singular common noun: “Jim’s car ran out of gas.” But if a proper noun ends in s, add an apostrophe only: “Lewis’ training regimen is impressive.”
If a plural noun ends in s, use an apostrophe only: “Cats’ claws are retractable.” If the plural does not end in s, use an apostrophe and an s: “The men’s room is to the right.”
If the possession applies jointly to two singular nouns, apply the apostrophe and s only to the latter noun: “Green eggs and ham’s popularity has declined.” (In this sense, “green eggs” is plural in construction but singular in meaning; it’s a menu item, not a grouping of eggs.) But if the two nouns are distinct, apply the appropriate possessive form for each noun to that noun: “My hat’s color and my shoes’ color match.”
Book style is simultaneously more complex and more consistent:
No exception is made depending on whether singular nouns preceding and following the apostrophe end and start with s, respectively: “The waitress’s smile was infectious.” (However, all plural nouns are treated the same as in AP style.)
Some ambiguity occurs, though, with traditional expressions following the “for (blank) sake” form: When the noun ends in an s, an apostrophe alone is employed (“For goodness’ sake, put on some clothes!”), while expressions with singular nouns follow the normal style: “For expedience’s sake, I faxed the form.” Better yet, in this case, relax the expression: “For the sake of expedience, I faxed the form.”
The primary difference in style between the two resources is that singular proper nouns are always followed by an apostrophe and an s, even if they end in s (or x or z). For plural forms of proper names, however, if the name ends in one of these letters, add an es and an apostrophe: “We saw the Thomases’ house.” Otherwise, add an s and an apostrophe only: “We found the Smiths’ cat.”
In the most recent edition of the manual, the style for two special cases has changed: Words ending in an unpronounced s are treated the same as words with a normal s sound (“Dubois’s translation is better”), as are words ending in an -eez sound (“Xerxes’s defeat was definitive”).
However, common nouns plural in form but singular in meaning take an apostrophe only: “Economics’ complexities are daunting.” The same is true of proper names: “The Rocky Mountains’ discovery by European explorers prompted exploitation of mineral and other natural resources.” However, many writers find these constructions awkward, and prefer to relax the constructions: “The complexities of economics complexities are daunting”; “The discovery of the Rocky Mountains by European explorers prompted exploitation of mineral and other natural resources.”
The distinction in AP style between joint possession and separate possession (see the last paragraph in the previous section) also applies to Chicago style.
Some other special cases include possessives with the genitive form (“Three hours’ delay made all the difference”); a possessive form of a phrase that appears attributive (“The farmers’ market has a wide selection of tomatoes,” but “I have a Diners Club card”); and plurals of publication and composition titles. In the case of such titles, if it is italicized, add an apostrophe and an s (“I read the New Yorker’s article about it,” but, because the following title ends in an s, “The New York Times’ editorial supports the bill.”) If quotation marks are called for, relax the construction: “The first line of ‘My Life in Verse’ appropriately describes her birth,” rather than “‘My Life in Verse’’s first line appropriately describes her birth.”)
Possessives followed by a gerund — a present participle (a type of verb ending in -ing) acting as a noun — should use an apostrophe and an s if singular (“The doctor’s telling of the story was interrupted”) and an apostrophe alone if plural (“Doctors’ taking on of more responsibility are a factor”). It might be better, however, to revise the sentence in such cases: “The doctor’s account of the story was interrupted”; “Quality of care is affected when doctors take on more responsibility.” Also, this grammatical structure should not be confused with the similar participle form, which is not possessive: “Doctors taking on more responsibility are likely to offer a lower quality of care.”
The apostrophe is optional when a noun (or a pronoun) follows a preposition: “They thought of their friends moving away as a personal affront” could also be rendered “They thought of their friends’ moving away as a personal affront,” where moving refers to an act of moving that “belongs” to their friends.