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Many questions already ask about this topic (What is the correct possessive for nouns ending in "‑s"? , When did it become correct to add an “s” to a singular possessive already ending in “‑s”?, etc.) and their answers vary, but they always give exceptions to the apostrophe-s rule, for example:

6.24 The general rule for the possessive of nouns covers most proper nouns, including most names ending in sibilants."

Examples they give include Kansas’s, Ross’s land, and Jones’s reputation. Exceptions include Jesus’  and Moses’.

Which names does this apply to? Is the Aeneas’  form correct, or is it Aeneas’s instead?

The most useful rule — and the most general and the easiest to remember — is simply that you add ’s whenever you actually say an extra /əz/ at the end when forming the possessive, compared with how you say the non-possessive version. Let your own ear be your guide. That’s all there is to it. No fancy rules full of exceptions. Just your own ear (as a native speaker, mind you).

So words ending in unstressed /iːz/ are exempt, like for example this series’ end, that species’ demise, Mercedes’, Ramses’, Sophocles’, Socrates’, Achilles’, Diomedes’, Archimedes’, Eratosthenes’, Ulysses’. (But not trapeze’s, because that one is stressed! See how that works?)

But these days, not much else is. I say “in these days” because in previous ages, some people did not add another /əz/ if it already had one, and so wrote Jesus’ to indicate they did not say an extra /əz/ there compared with Jesus: both are just /ˈd͡ʒiːzəs/ However, most people today now say Jesus’s, because it has three syllables: /ˈd͡ʒiːzəsəz/.

Same with Moses’s with three syllables instead of the older Moses’ with just two. Note that things like Ross’ and Chaz’ are always wrong, because no one says those with only a single syllable. That is a common error.

So it’s your boss’s house, because it’s got an extra syllable when you say it. Similarly, all the Jameses I have ever personally known have had the extra /əz/ tacked on when you are talking about something of theirs, which means it is for those speakers James’s house, albeit all the Jameses’ houses, because nouns are only allowed one /əz/ inflection, not two.

In all cases, the best thing to do is let your own ear be your guide, because writing should represent speech. That means that if you say an extra /əz/ then you write ’s, but if you don’t say it, then you don’t write it. That’s why you from time to time see forms like for goodness’ sake or for conscience’ sake. Those are possessive, but have no extra syllable.

As for the specific case of Aeneas, in older writing you will find that because his name already ends in /əz/, people would suppress the extra one when they would form the possessive, like Aeneas’ escape from Troy. Note very carefully that that posits a three-syllable possessive when spoken. If when you yourself say it, however, it turns out that you would yourself use the four-syllable version as most people today now do, then it would have to be Aeneas’s escape from Troy.

But now you have three issississes in a row, which will certainly require careful elocution to pull off — especially if you don’t mean to sound like Gollum with his fisheses.

Bryan Garner, Garner's Modern American Usage, second edition (2003) offers the following discussion of how to handle possessive proper names ending in -s:

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.

The Chicago Manual of Style, sixteenth edition (2010) reverses the guideline that its predecessors promoted in precisely the area that the poster asks about. Here are the relevant subsections of Chicago 16:

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[.]

...

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[.]

Having laid out those particulars, Chicago 16 also acknowledges an alternative rule that would reverse its main guideline on proper names:

7.21 An alternative practice for words ending in "s." Some writers and publishers prefer the system, formerly more common, of simply omitting the possessive s on all words ending in s—hence "Dylan Thomas' poetry," "Etta James' singing," and "that business' main concern." Though easy to apply and economical, such usage disregards pronunciation and is therefore not recommended by Chicago.

Of course, Chicago's new guideline 7.18 likewise disregards pronunciation—and yet it is not therefore not recommended by Chicago.

Words into Type, third edition (1974) offers these guidelines on "formation of the possessive case":

Proper names. The possessive form of almost all proper names is formed by adding apostrophe and s to a singular or apostrophe alone to a plural. [Relevant examples:} Jack's[;] James's[;] the Davises'[;] Burns's[;] Marx's[;] Schultz's[;] Dickens's[;] Adams's[;] Schultzes'[.] Wherever the apostrophe and s would make the word difficult to pronounce, as when a sibilant occurs before the last syllable, the apostrophe may be used alone. [Examples:] Moses' laws[;] Isis' temple[;] Xerxes' army[;] Jesus' followers[.] ... By convention, ancient classical names ending in s add only the apostrophe to form the possessive. [Examples:] Mars' wrath[;] Achilles' heel[;] Hercules' labors[.]

The problem with this treatment is that all of the "difficult pronunciation" examples involve vowel-s-vowel-s names (Moses, Isis, Jesus) and all of the "ancient classical names" examples involve mythological figures (Mars, Achilles, Hercules), so we don't have explicit guidance on whether Euripides (for example) would qualify as "difficult to pronounce" in possessive form (probably not) or as an "ancient classical name" (maybe).

The Associated Press Style Book and Briefing on Media Law (2002) endorses a rule that looks a lot like the "alternative practice" mentioned disapprovingly in Chicago's section 7.21:

SINGULAR PROPER NAMES ENDING IN S: Use only an apostrophe: Achilles' heel, Agnes' book, Ceres' rites, Descartes' theories, Dickens' novels, Euripides' dramas, Hercules' labors, Jesus' life, Jules' seat, Kansas's schools, Moses' law, Socrates' life, Tennessee Williams' plays, Xerxes' armies.

And finally, The Oxford Guide to Style (2002) has these relevant guidelines for forming possessives of proper names ending in -s:

• 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 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.

Conclusions

Style guides are all over the place on ho to handle the possessives of singular proper names—especially singular proper names from antiquity. In the specific instance of the possessive of Aeneas, this is approximately where the style guides I consulted come down:

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]

This list shows considerable support for Aeneas', but it is worth noticing that both Garner and the current Chicago—which carry considerable authority in publishing houses in the United States—favor Aeneas's. If you are supposed to follow one of these style guides, your course is clear; if not, you have some research to do (if some other authority controls your style choices) or a decision to make (if you're on your own).

The simplest rule is to always add ’s to a singular noun. It is traditional to omit it on a few words, but it's never wrong to add it. Since you don't have to pronounce the ’s, how the word sounds has nothing to do with it.