Generally, a possessive is formed by adding an apostrophe and an s to a word that does not end in s, and only an apostrophe to a word that does end in s.
Add an apostrophe to a word that ends in an s sound.
for old times’ sake
for conscience’ sake
for appearance’ sake
Add an apostrophe and an s to a foreign name ending in a silent sibilant.
Des Moines’s schools
Add an apostrophe and an s to the last word of a singular compound noun.
the Governor of Maine’s
the attorney general’s
Use an of phrase to show possession when both a plural and a possessive are involved in a compound noun.
RIGHT: the decisions of the attorneys general
WRONG: the attorneys general’s decisions
Indicate common possession by making only the last item in a series possessive.
Teddy, Peggy, and Nancy’s home
Indicate individual possession by making each item in a series possessive.
Teddy’s, Peggy’s, and Nancy’s homes
The following possessives should be written as singular per Web. 11. (updated 11/21/14)
The following possessive should be written as plural per Web. 11. (updated 11/21/14)
Consider that in some cases words are not possessive but rather descriptive. In those cases, no apostrophes are needed. See descriptive words for more detail. (added 12/3/14)