Apostrophe

An apostrophe (') is a punctuation mark with two main jobs: showing possession and forming contractions. In "the student's notebook," the apostrophe signals ownership; in "can't" or "they're," it marks the spot where letters were dropped when two words were squeezed together.

Possessive rules follow the noun's number. A singular noun takes 's (the boss's office), a plural noun ending in s takes only an apostrophe (the teachers' lounge), and an irregular plural that doesn't end in s takes 's (the children's toys). Singular proper nouns ending in s are style-dependent: Chicago style writes James's, while AP style writes James'. The placement of the apostrophe therefore tells you exactly how many owners there are: "my sister's cars" means one sister, while "my sisters' cars" means more than one.

The most tested trap is its versus it's. "It's" is always the contraction of "it is" or "it has," while "its" — with no apostrophe — is the possessive, matching other possessive pronouns like his, hers, and theirs, none of which take apostrophes. The other classic error is the "grocer's apostrophe": using an apostrophe to form a simple plural ("apple's for sale") is always wrong.

Apostrophe rules are heavily tested in the English and writing sections of the ACT, SAT, and CLT. Questions typically make you choose among its/it's, singular and plural possessive forms, and plain plurals — so the fastest strategy is to expand any candidate contraction ("it is") and count the owners before picking an answer.