Links in this essay will take you to information about the usage experts and their work. Numbers in parentheses are page references in the sources.
-'s for singular nouns ending in -s or an s sound
Is it Marcus' homework or Marcus's homework? Jesus' teachings or Jesus's teachings? Sophocles' plays or Sophocles's plays? Marx' theories or Marx's theories? The witness' testimony or the witness's testimony?
Some language experts apply a simple, across-the-board rule to form the possessive for a singular noun that ends in -s or an s sound: Always use the apostrophe plus -s or always use the apostrophe alone. But most experts come down somewhere in the middle.
The New York Public Library Writer's Guide to Style and Usage argues that "adding 's to a word that already ends in s creates an excess of s's" (268). Its examples of acceptable possessives for both common and proper nouns include "theboss' memos," "the hostess' chair," "James' idea," and "Charles Addams' cartoons" (268–69).
Roy Copperud belongs to the "if you speak it, add it" school of thought: "If you add an s sound in speaking the word in its possessive form, add apostrophe s; if the pronunciation is unchanged, add just the apostrophe" (303). The American Heritage Book of English Usage concurs (241).
Copperud considers boss' a misrepresentation, because the word is pronounced "boss-ez." At the same time, he objects to writing Moses's, because "you would not say Moseses, which is how Moses's would sound" (303).
But putting the pronunciation principle into practice can be complicated. Pronunciation varies from person to person. Indeed, sometimes a single person might pronounce a possessive differently depending on whether the word that comes next begins with another s sound. For instance, many people would say "Charles-ez car" but "Charles sports car." Nonetheless, anyone who writes Charles's car and Charles' sports car in the same piece of writing will appear inconsistent.
How should you treat a name that ends in an unpronounced -s? Should you write Camus' or Camus's, François' or François's? Among the experts cited in this book, only Fowler gives an unequivocal answer: Use -'s.
Bryan Garner lays out a clear-cut, easy-to-follow, centrist rule: "To form a singular possessive, add -'s to most singular nouns—even those ending in -s, -ss, and –x (hence Jones's, Nichols's, witness's, Vitex's)" (644). Patricia O’Conner concurs, giving as examples Paris’s, Degas’s, and Bordeaux’s (36). For reasons of tradition, Garner makes an exception for biblical or classical names such as Aristophanes' and Jesus', and he also excludes the names of companies or countries that end in -s: General Motors' profits, the United States' reputation.
Conclusion: Unless you are instructed to use a specific style, Garner's and O’Conner’s rule and Garner’s exceptions are straightforward and not likely to confuse you or your readers.
Source: Hacker Handbooks (Boston: Bedford, 2013).