POSSESSIVES. A. Singular Possessives. 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). ...
There are four exceptions to this rule: (1) The possessives of personal pronouns do not take apostrophes (ours, yours, its, theirs). ... (2) Biblical and Classical names that end with a /zəs/ or /eez/ sound take only the apostrophe: Aristophanes' plays[;] Jesus' suffering[;] Moses' discovery[;] Xerxes' writings[.] No extra syllable is added in sounding the possessive form. (3) If a corporate or similar name is formed from a plural word, it takes only the apostrophe. Thus General Motors makes General Motors', not General Motors's ... (4) According to traditional rules, a sibilant possessive before sake takes merely an apostrophe, without an additional -s—hence for appearance' sake, for goodness' sake, and for conscience' sake.
7.16 Possessives of proper nouns, letters, and numbers. The general rule [to add -'s to create possessive forms] extends to proper nouns, including names ending in s, x, or z, in both their singular and plural forms, as well as letters and numbers. SINGULAR FORMS [examples:] Kansas's legislature[;] Tacitus's histories[;] Chicago's lakefront[;] Borges's library[;] Marx's theories[;] Dickens's novels[;] Jesus's adherents[;] Malraux's masterpiece[;] Berlioz's works[;] Josquin des Prez's motets[.]
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7.18 Possessive of names like "Euripides." In a departure from earlier practice Chicago no longer recommends the traditional exception for proper classical names of two or more syllables that end in an eez sound. Such names form the possessive in the usual way (though when these forms are spoken, the additional s is generally not pronounced). [Examples:] Euripides's tragedies[;] the Ganges's source[;] Xerxes's armies[.]
Exceptions to the General Rule
7.19 Possessive of nouns plural in form, singular in meaning. When the singular form of a noun ending in s is the same as the plural (i.e., the plural is uninflected), the possessives of both are formed by the addition of an apostrophe only. ... [Examples:] politics' true meaning[;] economics' forerunners[;] this species first record (or, better, the first record of this species)[.]
The same rule applies when the name of a place or an organization or a publication (or the last element in the name) is a plural form ending in s, such as United States, even though the entity is singular. [Examples:] the United States' role in international law[;] Highland Hills' late mayor[;] Callaway Gardens' former curator[;] the National Academy od Sciences' new policy[.]
7.20 "For ... sake" expressions. For the sake of euphony, a few for ... sake expressions used with a singular noun that ends in s end in an apostrophe alone, omitting the additional s. [Examples:] for goodness' sake[;] for righteousness' sake[.] Aside from these traditional formulations, however, the possessive in for ... sake expressions may be formed in the normal way. [Examples:] for experience's sake[;] for appearance's sake (or for appearances' sake {plural possessive} or for the sake of appearance)[;] for Jesus's sake[.]
- Use 's after non-classical or non-classicizing personal names ending in an s or z sound: Charles's[;] Marx's[;] Dicken's[;] Leibnitz's[;] Onassis's[;] Zachariasis[;] Collins's[;] Tobias's[.] While convention allows latitude in possessives (e.g. the additional s is used more in speech than in writing), the possessive misconstruction Charles Dicken's is always incorrect.
- An apostrophe alone is also permissible in after longer non-classical or non-classicizing names that are not accented on the last or penultimate syllable: Nicholas'(s)[;] Barnabas'(s)[;] Augustus'(s)[.] Jesus's is acceptable in non-liturgical use. Jesus' is an accepted archaism—Good friend for Jesus' sake forbear—and Jesu's is also possible in older contexts.
Use 's after non-classical or non-classicizing personal names ending in an s or z sound: Charles's[;] Marx's[;] Dicken's[;] Leibnitz's[;] Onassis's[;] Zachariasis[;] Collins's[;] Tobias's[.] While convention allows latitude in possessives (e.g. the additional s is used more in speech than in writing), the possessive misconstruction Charles Dicken's is always incorrect.
Use an apostrophe alone after classical or classicizing names ending in s or es: Arsaces'[;] Ceres'[;] Demosthenes'[;] Euripides'[;] Herodotus'[;] Mars'[;] Miltiades'[;] Themistocles'[;] Venus'[;] Xerxes'[;] Erasmus'[;] Philip Augustus'[.] This traditional practice in classical works is still employed by many scholars. Certainly follow it for longer names (though Zeus's, for instance is possible), as well as for post-classical Latinate names favoured throughout the Middle Ages. ...
- Use 's after French names ending in silent s or x, when used possessively in English: Dumas's[;] Descartes's[;] Hanotaux's[;] Crémieux's[;] Lorilleux's[.] However, since appending the plural s would be grotesque (Lorilleuxs) or misleading (Dumass), the singular possessive is treated like the plural, for example both Lorilleux's (not Lorilleuxs') cat, the two Dumas's (not Dumass') novels.
Use 's after French names ending in silent s or x, when used possessively in English: Dumas's[;] Descartes's[;] Hanotaux's[;] Crémieux's[;] Lorilleux's[.] However, since appending the plural s would be grotesque (Lorilleuxs) or misleading (Dumass), the singular possessive is treated like the plural, for example both Lorilleux's (not Lorilleuxs') cat, the two Dumas's (not Dumass') novels.
Garner's Modern American Usage: Aeneas's [because, although it is a classical name, it ends with neither a /zəs/ nor a /eez/ sound; instead it ends with /əs/]
Chicago Manual of Style, fifteenth edition and earlier: Aeneas' [because that is the traditional handling of the possessive for classical names]
Chicago Manual of Style, sixteenth edition: Aeneas's [because Chicago 16 breaks with the guideline in previous editions on this point]
Words into Type: Aeneas' [because Aeneas is an ancient classical name ending in -s]
Associated Press Style Book and Briefing on Media Law: Aeneas' [because that's AP's standard approach to any singular proper name ending in -s]
The Oxford Guide to Style: Aeneas' [because Aeneas is a classical or classicizing name ending in -s or -es]
