• Add an apostrophe and an s ('s) to words that don't end with an s.

somebody's car (the car belonging to somebody)
the men's locker room (the locker room of the men)
a month's rent (the rent of a month)

• Add only an apostrophe after the to words that already end in s.

twelve days' pay (that is, the pay for twelve days)
the students' tests (that is, tests belonging to the students)

• To show joint possession, add an apostrophe and s to only the last word in the
group: e.g.,

Laurel and Hardy's last movie (a movie by the team of Laurel and Hardy.)

• But to show individual possession, add an apostrophe and s to each word in the
group: e.g.,

Fossum's and Day's opinions (the opinions of Fossum, and the opinions of Day)

To Form Contractions

Show Omission of Letters with Apostrophes

Could not = couldn't He is = he's They are = they're Cannot = can’t

Correct an Especially Troublesome Error

It’s (a contraction) means it is (e.g., It’s not snowing now).
Its means belonging to it (e.g., The school lost its funding).

To test for correctness, ask yourself, “Do I mean it is?”

Avoid Misuse of apostrophes.

Do not use an apostrophe for simple plurals.

Wrong: All the player's knew this was the last chance to score.
Right: All the players knew this was the last chance to score.

Do not use an apostrophe for pronouns that already indicate possession.

Wrong: We didn't know the suitcases were her's (your's, their's, our's).
Right: We didn't know the suitcases were hers (yours, theirs, ours).

In APA style, do not use apostrophes for plurals of numerals or letters.