This post outlines the prevailing rules and recommendations for employing apostrophes when using the possessive form of a noun and discusses in which cases an s should follow the apostrophe.
Apostrophes are used to indicate singular possession, as in “The dog’s collar is too tight,” and plural possession, as in “Several of our neighbors’ cars were vandalized.”
When referring to two or more people collectively, use an apostrophe only after the last noun or name: “John and Jane’s daughter is going off to college.” When referring to two or more people separately, employ apostrophes for each noun or name: “My doctor’s and dentist’s names are the same.”
Some style handbooks recommend using only an apostrophe after singular nouns and proper names ending in s, as in “The witness’ last statement is puzzling” and “He is among the most eccentric of Dickens’ characters,” but this style is prevalent primarily in journalistic writing, and most style guides call for an additional s: “The witness’s last statement is puzzling” and “He is among the most eccentric of Dickens’s characters.” However, plural possessive forms of surnames should be treated as in “The Smiths’ house is the third one on the right.” (Plural possessive forms of names ending in s are treated as in “We had dinner at the Thomases’ house.”)
An exception used to be made for words and names ending in an unpronounced s as well as biblical or classical names ending in s, but now it is recommended that these be supplied with an additional s; examples include the names in “Descartes’s treatise” and “Jesus’s followers.”
However, when the singular and plural forms of a noun are the same, omit the final s, as in “The species’ distinguishing characteristics are listed below.” When the name of an entity such as a city ends in s but is singular, likewise, use an apostrophe only: “Construction of El Dorado Hills’ new community center is underway.” If such conflicting usage seems awkward, avoid the possessive form; instead, write “The distinguishing characteristics of the species are listed below” and “Construction of the new community center in El Dorado Hills is underway.”
When using an idiom beginning with for and ending in sake, such as “for goodness’ sake,” omit the final s.
Apostrophes are also used in the genitive case, in expressions such as “two weeks’ notice,” and in possessive forms that resemble the attributive use of a noun (that is, a noun modifying another noun), such as “farmers’ market,” meaning “a market belonging to farmers.” Some people choose to style such phrases attributively (“farmers market,” meaning “a market of farmers”), but such use is best reserved only for proper names (for example, “the Department of Veterans Affairs”).
When a gerund follows a noun, the noun should be treated possessively, as in “Doctors’ prescribing such medication is problematic,” meaning “The habit among doctors of prescribing such medication is problematic,” but perhaps it is better to simply use the alternative wording. The possessive form of a noun that follows a preposition, however, is optional; one may write either “They knew about their supervisor’s spying on them” or “They knew about their supervisor spying on them,” but this, too, is perhaps better revised to “They knew that their supervisor was spying on them.”
Italicized publication and book titles should be followed by an nonitalicized apostrophe and s, as in “People’s cover story” and “War and Peace’s formidable length,” though periodical titles ending in s should be followed by an apostrophe only, as in “the Los Angeles Times’ subscription data.”