An apostrophe (’) has three functions: to show ownership or possession, to indicate omitted letters in contractions, and to form some plurals.

Add -’s to make a singular noun possessive, including nouns that end with s or the sound of s and indefinite pronouns (anyone, nobody).

Note that the possessive forms of personal pronouns do not take apostrophes: mine, yours, his, hers, ours, theirs, its.

The possessive form of who is whose (not who’s).

Add an apostrophe to a plural noun to make it possessive, or add -’s if the plural noun does not end in s.

To show individual possession by two or more people or groups, add an apostrophe or -’s to each noun.

Sam works for two different teams.

To show joint possession by two people or groups, add an apostrophe or -’s to the last noun.

Add -’s to the last word of a compound noun to show possession.

In the sentences above, note that numbers, letters, and words used as themselves are in italics. The -s ending should not be italicized, however. (For more on italics and underlining, see Section 23.)

When referring to the years in a decade, no apostrophe is used.

Apostrophes are used to signal the omission of the numerals that indicate the century.