Rules for writers, Tenth edition - Diana Hacker, Nancy Sommers 2021

Use an apostrophe to indicate that a noun is possessive
The apostrophe
Punctuation

Possessive nouns usually indicate ownership, as in Tim’s hat or the lawyer’s desk. Frequently, however, ownership is only loosely implied: the tree’s roots, a day’s work. If you are not sure whether a noun is possessive, try turning it into an of phrase: the roots of the tree, the work of a day. (Pronouns also have possessive forms. See 37b and 37e.)

When to add -’s

1. If the noun does not end in -s, add -’s.

o Luck often propels a rock musician’s career.

o The Children’s Defense Fund is a nonprofit organization that supports programs for poor children.

2. If the noun is singular and ends in -s or an s sound, add -’s to indicate possession.

o Lois’s sister spent a year in India.

o Her article presents an overview of Marx’s teachings.

NOTE: To avoid potentially awkward pronunciation, some writers use only the apostrophe with a singular noun ending in -s: Sophocles’.

When to add only an apostrophe

If the noun is plural and ends in -s, add only an apostrophe.

Both diplomats’ briefcases were searched by guards.

Joint possession

To show joint possession, use -’s or (-s’ ) with the last noun only; to show individual possession, make all nouns possessive.

Have you seen Joyce and Greg’s new camper?

Hernando’s and Maria’s expectations of marriage couldn’t have been more different.

Joyce and Greg jointly own one camper. Hernando and Maria individually have different expectations.

Compound nouns

If a noun is compound, use -’s (or -s’ ) with the last element.

My father-in-law’s memoir about his childhood in Sri Lanka was published in September.