An apostrophe does two main jobs: it shows possession (the cat's paw), and it replaces the letters left out of a contraction (can't). It never makes a plain plural: two cats, not two cat's.
Apostrophes seem to be a part of punctuation that many people simply do not understand. The apostrophe is left out, put in the wrong place, or used where it does not belong at all. And the problem goes much further than the well-known mix-up of your and you're.
Here are some simple rules and examples to get rid of the headaches that apostrophes tend to cause.
Possession
One job of the apostrophe is to show possession. Usually, you show possession by adding an apostrophe and an s:
- the chef's hat
- the cat's paw
It gets harder when the word already ends in s. Here there is no single fixed rule. A method that many newspapers use is to add an apostrophe and s to common nouns that end in s, and only an apostrophe to names that end in s:
- the class's size
- Mrs. Jones' cat
Other style guides, such as The Chicago Manual of Style, add the apostrophe and s to names as well: Mrs. Jones's cat. Whichever you choose, use it consistently.
Plural nouns that end in s, which is most of them, take the apostrophe only:
- Employees' entrance only.
- the two cats' owner
Plurals that do not end in s take an apostrophe and s, just like singular nouns:
- the children's toys
- the women's team
When a name that ends in s is made plural with -es, add the apostrophe after the plural ending:
- the Joneses' cat
Put the apostrophe in the right place
Be careful not to put the apostrophe in the wrong place in the word. This happens often, especially with plurals. If you write the cat's scratching post when you mean the cats' scratching post, you are talking about a single cat and not two or more, which changes the meaning of the sentence.
Likewise, apostrophes are not used to make the plural of a noun. "I own two cat's" is wrong, and so is "We met the Jone's" when you mean a family named Jones. The correct forms are "I own two cats" and "We met the Joneses." (Plurals of single letters, as in mind your p's and q's, are a special case; see plurals with apostrophes.)
Compound nouns
In compound nouns, the apostrophe and s go on the last word. If the compound is plural, make it plural first:
- my mother-in-law's cat
- my sisters-in-law's husbands
Contractions
In a contraction, the apostrophe replaces the letters that have been left out:
- can't (cannot)
- wouldn't (would not)
- shouldn't (should not)
- you're (you are)
Other uses
An apostrophe also marks letters or digits left out at the beginning of a word or number:
- I got my first cat back in '03.
- rock 'n' roll
Here it should not be mistaken for an opening single quotation mark. Word processors often turn it into one, which curls the wrong way. An apostrophe always has the shape of a closing quotation mark, like a small raised 9.
Watch out for these
The possessive pronouns never take an apostrophe: his, hers, its, ours, yours, theirs, and whose. It's always means "it is" or "it has" (more in its vs. it's).
Also, a word that ends in -y changes the y to ie in the plural, but not in the singular possessive:
- the lady's cat (one lady)
- the ladies' cats (several ladies)