A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Possessives - General rule
Spelling
Part III. Style 20 spelling

Form the possessive of most singular common and proper nouns, including those that end in s, x, or z, by adding an apostrophe and s. This rule also applies to letters and numerals used as singular nouns, and to abbreviations. (Do not confuse possessives with plural forms, which are described in 20.1). For special cases, see 20.2.2.

an argument's effects

Stevens's poems

2009's economic outlook

the phalanx's advance

Diaz's revolt

JFK's speech

Inanimate nouns—except for references to time—rarely take the possessive form.

a day's length

but not

the house's door

Form the possessive of most plural common and proper nouns by adding only an apostrophe. For special cases, see 20.2.2.

politicians' votes

not

politicians's votes

the Rodriguezes' house

not

the Rodriguezes's house

For irregular plurals that do not end in s, add s after the apostrophe.

the mice's nest

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