Here are several rules for the possessive case.
A. Most singular nouns form their possessive by adding an apostrophe
and an s. (the baboon’s food; the girl’s sweater; Marx’s teachings; Burns’s
poetic lines; Dickens’s characters)
B. To form the possessive of a singular noun that ends with an s sound,
take one of two actions.
1. If a name of two or more syllables ends in an eez sound, the possessive
is formed without an additional s. (Ulysses’ friends; Archimedes’
2. Add an apostrophe and an s if the word would not be difficult to
pronounce. (dress’s cost; quartz’s essence)
C. Add only an apostrophe to form the possessive of a plural noun that
ends in s. (the boys’ gymnasium; the Murphys’ home)
D. If a plural noun does not end in s, add an apostrophe and an s. (the
men’s department; the mice’s hiding spots)
E. Use the possessive form for only the last name in compound nouns for
organizations, literary titles, businesses, and relatives. If owned separately,
use the possessive for both names.
Tom’s and Pete’s reputations (separate reputations)
Procter and Gamble’s sales (combined ownership)
mother-in-law’s magazines (one woman’s ownership)
mothers-in-law’s magazines (two or more women’s ownership)
F. For acronyms (words formed from the first letters of a series of words),
add an apostrophe and an s.
the NHL’s (National Hockey League’s) members
AARP’s (American Association of Retired People’s) membership