Pursuant to earlier discussion, I have looked at some different books of grammar 'rules' and find inconsistency.
Fowler's
Use 's for the possessive case in English names and surnames whenever possible; i.e. in all monosyllables and disyllables, and in longer words accented on the penult, as Burns's, Charles's, Cousins's, Dickens's, Hicks's, St James's Square, Thomas's, Zacharias's. It is customary, however, to omit the 's when the last syllable of the name is pronounced /-IZ/, as in Bridges', Moses'. Jesus' is an acceptable liturgical archaism.
In ancient classical names use s' (not s's): Mars', Herodotus', Venus'. Ancient names written -es are usually written -es' in the possessive: Ceres' rites, Xerxes' fleet. In longer words of this type, -es' should also be used: Demosthenes', Euripides', Socrates', Themistocles'.
Normally, I rely on Fowler's, but I must say this is rather messy.
Next, Modern Language Association's
MLA Style Manual:
To form the possessive of a plural noun ending in s, add only an apostrophe: photographers' exhibit
but
To form the possessive of any singular proper noun, add an apostrophe and an s: Dickens's reputation, Descartes's philosophy, Marx's precepts, 'Venus's beauty
but still
To form the possessive of a plural proper noun, add only an apostrophe: the Dickenses' economic woes, the Vanderbilts' estate
A wee bit simpler, but still messy.
Finally, the
Chicago Manual of Style7.17 Most nouns. The possessive of most singular nouns is formed by adding an apostrophe and an s, and the possessive of plural nouns (except for a few irregular plurals that do not end in s) by adding an apostrophe only. This practice, used in conjunction with the exceptions and options outlined in 7.19-22, reflects the way possessive forms are generally pronounced and is largely faithful to Strunk and White's famous rule 1 ("Form the possessive singular of nouns by adding 's"). Since feelings on these matters sometimes run high, users of this manual may wish to modify or add to the exceptions. For an alternative practice, see 7.23. ...
the horses' mouth
a bass's stripes
puppies' paws
children's literature
a herd of sheep's mysterious disappearance
7.18 Proper nouns, letters, and numbers. The general rule covers most proper nouns, including names ending in s, x, or z, in both their singular and plural forms, as well as letters and numbers.
Kansas's legislature the Williamses' new house
Chicago's waterfront Malraux's masterpiece
Burns's poems Inez's diary
Marx's theories the Martinezes' daughter
Berlioz's works Josquin des Prez's motets
Strauss's Vienna dinner at the Browns' (that is, at the Browns' home)
Dickens's novels FDR's legacy
the Lincolns' marriage 1999's heaviest snowstorm
William's reputation
The rule applies equally to company names that include a punctuation point.
Yahoo!'s chief executive
7.19 Nouns plural in form, singular in meaning When the singular form of a noun ending in s looks like a plural and the plural form is the same as the singular, the possessive of both singular and plural is formed by the addition of an apostrophe only. If ambiguity threatens, use of to avoid the possessive.
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 the last element in the name) is a plural form ending in s such as the United States, even though the entity is singular.
the United States' role in international law
Highland Hills' late mayor
Calloway Gardens' former curator
the National Academy of Sciences' new policy
7.20 Names like "Euripides." The possessive is formed without an additional s for a name of two or more syllables that ends in an eez sound.
Euripides' tragedies the Ganges' source Xerxes' armies
7.21 Words and names ending in unpronounced "s." To avoid an awkward appearance, an apostrophe without an s may be used for the possessive of singular words and names ending in an unpronounced s. Opt for this practice only if you are comfortable with it and are certain that the s is indeed unpronouned.
Descartes' three dreams
the marquis' mother
François' efforts to learn English
Vaucouleurs' asistance to Joan of Arc
Albert Camus' novels (the s is unpronounced)
but
Raoul Camus's anthology (the s is pronounced)
7.22 Other exceptions. For ... sake expressions traditionally omit the s when the noun ends in an s or an s sound.
for righteousness' sake
for goodness' sake
for Jesus' sake
but
Jesus's contemporaries
Where neither an s nor an apostrophe alone looks right (as with such names as Isis), avoid the possessive and use of instead.
And if this whole thing wasn't sufficiently confusing (Fowler's is tame by comparison), the last exception is a winner:
7.23 An alternative practice. Those uncomfortable with the rules, exceptions, and options outlined above may prefer the system, formerly more common, of simply omitting the possessive s on all words ending in s--hence "Dylan Thomas' poetry," "Maria Callas' singing," and "that business' main concern." Though easy to apply, that usage disregards pronunciation and thus seems unnatural to many.
My apologies for any typos.
Methinks this can be summed up with 'do whatever you feel like doing' -- because you can find some 'authority' to support you.
