SUNY Empire State College
Empire State College: Apostrophes
This resource looks at the three main uses of apostrophes: to take the place of deleted letters; to show possession; and to show plurals of letters, numbers, symbols, abbreviations and words used as words.
Other
Bellevue Community College: Apostrophes [Pdf]
Resource explains the three uses of the apostrophe and provides examples. Requires Adobe Reader. [PDF]