Apostrophe

An apostrophe' ('' or ’) is a punctuation mark in languages written in the Latin alphabet. It marks omissions, forms the possessive, and, in special cases, forms plurals.

Usage

An apostrophe is commonly used to indicate omitted letters as in:
- abbreviations, as gov't for government
- contractions, such as can't from cannot and it's from it is.
It is used with an added s to indicate possession, as in Oliver's army, James's crown.
It can be used to form a plural for abbreviations and symbols where adding just s rather than ’s would be ambiguous, such as mind your p's and q's. It isn't necessary where there isn't any ambiguity, so CDs not CD's, videos not video's, 1960s not 1960's.

Things to note

The apostrophe in it's marks a contraction of it is. The possessive its has no apostrophe. Many find this confusing. (It might help to remember that there is no apostrophe in any of his, hers, or its.)
Likewise, its role in pluralization of symbols has led to a modern tendency to use the apostrophe incorrectly to form plurals of words, that is plural's of word's.
When the noun is plural and already ends in s, no extra s is added, so pens' lids (where there is more than one pen) rather than pens's lids. If the plural noun doesn't end in s, then add 's as usual: children's hats.
If a name already ends with an s, the extra s is sometimes dropped: Jesus' parables. This is more common in U.S. usage and with classical names (Eros' statue, Herodotus' book).
Who's means who is. The possessive of who is whose. "The person whose responsibility it is is the member who's oldest."

Tip

To check you've got it right, swap the sentence around so that the part before the apostrophe becomes the last word. If the sense hasn't changed, you've got it right.

Pens' lids becomes lids of the pens.
Boy's hats becomes hats of the boy.
Boys' hats becomes hats of the boys.
Children's hats becomes hats of the children.
Two weeks' notice becomes notice of two weeks.
One week's notice becomes notice of one week.

But childrens' hats becomes hats of the childrens, so must be wrong.

In computer programming, the non-leaning apostrophe ('') corresponds to Unicode and ASCII character 39, or 0x0027. The (preferred) right-leaning apostrophe ('&#8217) corresponds to Unicode character 0x2019.