English Plus+
Grammar Slammer's Apostrophes Showing Possession
This site discusses the correct and incorrect usage of apostrophes to show possession and examples are provided. Note that links are provided at the bottom of the page for other specific uses of the apostrophe.
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Illinois State U.: Punctuation Made Simple Apostrophes
What is an apostrophe and how is it used properly in punctuation? "Punctuation Made Simple," gives some concrete examples for apostrophe use.