Apostrophe

1. Indicates the possessive case of singular and plural nouns, indefinite pronouns, and surnames combined with designations such as Jr., Sr., and II: my sister's husband, my three sisters' husbands, anyone's guess, They answer each other's phones, John Smith, Jr.'s car.
2. Indicates joint possession when used with the last of two or more nouns in a series: Doe and Roe's report.
3. Indicates individual possession or authorship when used with each of two or more nouns in a series: Smith's, Roe's, and Doe's reports.
4. Indicates the plurals of words, letters, and figures used as such: 60's and 70's; x's, y's, and z's.
5. Indicates omission of letters in contractions: aren't, that's, o'clock.
6. Indicates omission of figures in dates: the class of '63.

See also: Apostrophes.