How to Use an Apostrophe

There are 3 circumstances under which apostrophes must be used:

• Possessive forms of common and proper nouns: the cat's tail, John's house, the cats' tails, children's toys
• Indicating missing letters in contractions: It's me, Blizzard of '78, eight o'clock
• In certain proper names that are spelled with apostrophes: O'Brien, Xi'an

Additionally, the following usage is frowned upon, but begrudgingly accepted by ApostrophePatrol:

• (Marginal) Pluralizing nouns which are not actually words: DVD's, 1990's.

In all other cases, apostrophes must not be used.