Grammar Smart 3rd Edition - Princeton Review 2014

I. The Apostrophe
PART 4. Punctuation

The apostrophe is used to show ownership. Most of the time, it presents no confusion: Bob’s bassoon, the woman’s finger. The tricky part is using an apostrophe when the owner is plural.

Rules For Apostrophes

1. If the plural noun doesn’t end in -s, add an apostrophe and -s. (This is the easy part.)

the women’s fingers

the bacteria’s growth

the cat’s hairballs

2. If the plural ends in -s, just add an apostrophe.

the babies’ bottoms

the horses’ hooves

the politicians’ promises

3. If the word is a proper noun that ends in -s, add an apostrophe and an -s. (This is the part people get wrong.)

Yeats’s poem

Ross’s riddle

Chris’s crisis

4. One exception is the possessive of the pronoun it, which is its (no apostrophe). The word it’s (with apostrophe) is the contraction for it is, and not the possessive.

We’re giving the robot its weekly check-up today.

Not: We’re giving the robot it’s weekly check-up today.

It’s a beautiful day in the neighborhood.

Not: Its a beautiful day in the neighborhood.

If this strikes you as confusing, notice that the word it is treated similarly to the words he, she, and they. When apostrophes are added to these words, they become contractions: he’s going to the store, she’s going to bed, and they’re going to work. The possessive pronouns do not contain apostrophes: his book, her food, their table, its mountains.