Apostrophes have two uses: to indicate possession and to indicate an omission, as in a contraction.

Possession:

Apart from personal pronouns, possession is indicated by adding apostrophe s to

  • a singular noun, or
  • a one-syllable noun that ends in s, or
  • a plural noun that doesn’t end in s

Ms Goulet’s book; Jane’s handout; James’s memory stick

An apostrophe alone is added to

  • plural nouns ending in s, or
  • singular nouns of more than one syllable that end in s

the publishers’ convention; Mr. Jenkins’ book

Omission/Contractions

Where’er you omit a letter, an apostrophe shows the gap. Words like cant or wont can’t and won’t be confused if you are scrupulous about apostrophes.

In academic writing, contractions are often discouraged, so apostrophes will not appear except for possession or in quotations.

It’s and its

If you never use contractions, you’ll never be wrong about the frequently confused pair:

  • It’s = it is (a contraction)
  • Its = belonging to it

Finally, abbreviations are not an exception

PCs are not better than Macs. A PC’s operating system…

Use s alone to make an abbreviation plural; use apostrophe s to indicate possession.