The apostrophe. It's a small punctuation mark, but its misuse can cause huge problems in your writing. Here are some rules to guide you as you tell 'em whose it is.
Rule 1: To form the possessive of a singular noun that does not end in s or an s sound, add an apostrophe plus s to the noun:
Examples: the doctor's orders, the writer's desk, Tammy's car, my sister's children, her father-in-law's house
Rule 2: To form the possessive of a singular noun that does end in s or an s sound, add an apostrophe plus s to the noun:
Examples: Jennifer Lopez's music, the witness's report, James's poetry
One exception to this rule is to add only an apostrophe when adding the apostrophe plus s makes the word difficult to pronounce:
Examples: Sophocles' plays, Mrs. Rogers' new car
Rule 3: To form the possessive of a plural noun that ends in s or es, add only an apostrophe to the noun:
Examples: the actors' roles, the writers' convention, their doctors' orders, the beaches' cleanliness
Rule 4: To form the possessive of a plural noun that does not end in s or es, add an apostrophe plus s to the noun:
Examples: her children's toys, the women's dressing room
Rule 5: To indicate separate possession, add whichever possessive sign is appropriate (an apostrophe plus s or an apostrophe alone) to the name of each person:
Examples: Bill's and Tom's cars (two separate cars: Bill's car and Tom's car), James's and Olivia's houses (two separate houses: James's house and Olivia's house)
Rule 6: To indicate joint possession, add the appropriate possessive sign (an apostrophe plus s or an apostrophe alone) to the final name:
Examples: Mary and John's house (the house belongs to both Mary and John), Edward and Madeleine's books (the books belong to both Edward and Madeleine)
One exception to this rule occurs if one of the owners is identified by a pronoun (my, his, her, our, their). In this case, make each name and pronoun possessive:
Example: Erica's and my project (not Erica and my project); Mark's and our dinner (not Mark and our dinner); John's, Edgar's, Lisa's, and my party (not John, Edgar, Lisa, and my party)
A note about the possessive pronouns: my, mine, your, yours, his, her, hers, its, our, ours, their, theirs, whose. These pronouns never require apostrophes.
Rule 7: To form the possessive of a singular abbreviation, add an apostrophe plus s.
Examples: the FAA's ruling, the MD's diagnosis, USA's stand
Rule 8: To form the possessive of a plural abbreviation, add an s'.
Examples: the PhDs' dissertations, the RNs' orders, the CPAs' convention
A note about apostrophes in contractions: Whenever a word or phrase is shortened by contraction, remember to place an apostrophe at the point where the letters are omitted.
Examples: don't (do not), can't (cannot), rock 'n' roll (rock and roll), ma'am (madam)
Apostrophe placement shouldn't give you any trouble if you just follow these simple rules. So when Maria parks her car in Tom and Jane's driveway, which is next to the Petersons' house and across the street from Edward's and Alfred's apartments, you'll have no trouble at all conveying that information to your reader!