Use an apostrophe to denote ownership to a singular or plural noun and indefinite pronoun by adding an -'s if the word doesn't end in -s.

Of all forms of punctuation, the apostrophe appears to be in greatest peril of extinction. For proof that the apostrophe should be placed on an endangered species list in some grammarian's office, one needs only to consult the popular press or a sample of student themes. However, because of its ability to denote ownership in a concise way (by avoiding the use of a preposition), the apostrophe plays an important role in the English language. Despite the frequency of its misuse, the apostrophe is a fairly simple form of punctuation to master. You can denote ownership to a singular or plural noun and indefinite pronoun by adding an -'s if the word doesn't end in -s:

  • They worked on Susan's computer.
  • The children's toys are clustering the house again.
  • You expect me to do a week's worth of typing in two days.
  • The people's choice was Bill Clinton.
  • When it is someone else's turn to have his or her writing critiqued by the group, remember to be conscientious.

When a singular noun ends in -s, traditional grammarians recommend adding an -'s:

  • She loves Keats's poems.
  • The business's direct-mailing campaign worked wonders.
  • John Adams's letters illustrate his reflective spirit.

However, this usage can be cumbersome. Consequently, the following usage is also correct:

  • John Keats' conscience and life spirit energize his poems.
  • John Adams' ideas are worth examination.

When a plural noun ends in -s, you only need to add an apostrophe:

  • The judge confiscated the drivers' licenses.
  • She won two months' free groceries.
  • The writers' guild meets Monday.

With compound subjects, when you wish to denote individual ownership, you should add an -'s to each noun:

  • Dr. Wilson's and Mr. Speinberg's lawsuits were caused by poor communication.

Or, you can demonstrate joint ownership by placing the -'s after the second subject:

  • Pat and Joe's new car is hot!
  • This is my father-in-law and mother-in-law's office.

Finally, always use an apostrophe when you form a contraction. The apostrophe is positioned where letters are dropped:

it is it's
they are they're
you are you're

who is

who's

can not

can't

she will

she'll

were not

weren't