Possessive Rules of English Grammar
Part of the English Grammar Workbook For Dummies
Who owns what? An apostrophe helps answer that question. Here's how to place an apostrophe in the right spot to show possession:
Tack an apostrophe to a plural noun that ends in the letter s to create a possessive.
Add an apostrophe and the letter s to a plural noun that doesn't end in the letter s.
Possessive pronouns (my, his, theirs, whose, and so forth) never contain apostrophes.
Place a possessive noun or pronoun in front of an -ing verb form that is being used as a noun (my swimming, Arthur's dancing, and the like).