Apostrophes signal one of two things: contraction or possession.

Contractions squash two words together into one (or shorten one long word):

  • we will becomes we’ll
  • cannot becomes can’t
  • you are becomes you’re
  • it is becomes it’s
  • will not becomes won’t
  • is not becomes isn’t

Contractions are common in casual speech and writing, but try to avoid them in formal documents such as essays, reports, etc.

Using apostrophes to show possession—a sense of belonging—can be tricky. To singular nouns, add an apostrophe plus an s (even if the noun ends in s):

  • The book’s cover is torn.
  • Charles’s classes are dull. (Charles’ classes is also acceptable).
  • I ate my housemate’s leftovers.

But what if you were very hungry and helped yourself to all of the leftovers in the fridge? To show possession for a plural noun that ends in s—let’s assume you have several housemates—add an apostrophe after the s:

  • I ate my housemates’ leftovers.

For plural nouns that don’t end in s, add an apostrophe plus an s:

  • The children’s toys are broken.
  • The women’s washroom is on the left.

The exception to the rules of apostrophe use and possession is it. The possessive of it is its.

  • Incorrect: It’s collar was too loose and kept coming off.
  • Correct: Its collar was too loose and kept coming off.

Apostrophes are never used to pluralize standard English words, but they can be used to signal the plural of non-standard words such as acronyms (CD’s), single letters (p’s and q’s), or dates (the 1980’s). Please note: some usage guides omit the apostrophe in these cases (CDs, 1980s, etc.). Just pick a strategy and stick with it for consistency. You can also take your cue from other writing in your field, or your citation style guide.