A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007
Special cases
Spelling
Part III. Style 20 spelling
SINGULAR NOUNS ENDING IN S. Form the possessive of the following types of nouns with only an apostrophe:
nouns that name a group or collective entity but are treated as grammatically singular
politics' true meaning
the United States' role
names that end in an unpronounced s
Descartes' Discourse on Method
Albert Camus' novels
names of more than one syllable with an unaccented ending pronounced eez, including many Greek and hellenized names
Aristophanes' plays
the Ganges' source
Charles Yerkes' ideas
nouns in For . . . sake expressions that end in an s or an s sound
for conscience' sake
for appearance' sake
You might instead rephrase a sentence that contains one of these exceptions. (For use of the abbreviation U.S. in this situation, see 24.3.1.)
the role of the United States
instead of
the United States' role
for the sake of appearance
instead of
for appearance' sake
COMPOUND WORDS. Form the possessives of singular compound words by adding an apostrophe and s to the last word, even if the main noun is first.
his sister-in-law's business
the attorney general's decision
Avoid using the possessive for plural compounds in which the noun is followed by a prepositional phrase or adjective; rephrase instead.
decisions of the attorneys general
not
attorneys' general decisions
MULTIPLE NOUNS. If a possessive indicates that two or more entities each possess something separately, make all the nouns possessive.
New York's and Chicago's teams
historians' and economists' methods
If a possessive indicates that two or more entities possess something jointly, make only the last noun possessive.
Minneapolis and St. Paul's teams
historians and economists' data
TERMS IN ITALICS AND QUOTATION MARKS. If a term in italics is possessive, both the apostrophe and the s should be in roman type. Do not add a possessive to a term in quotation marks; rephrase the sentence.
the Atlantic Monthly's editor
admirers of “Ode on a Grecian Urn”