The apostrophe (’) joined with an “s” designates possessives—before the “s” in singular form and after the “s” in plural form. The apostrophe also represents omitted letters in contractions. [EDIT]
Writers should take care with possessive forms and with contractions. The use of the apostrophe for possessive forms appears to be disappearing in popular culture (Note the spelling on restrooms: “Mens” and “Womens” signs are becoming common, although the omission of the apostrophe is nonstandard.
Examples:
The student’s book. . . (singular) v. The students’ books. . . (plural)
You all = y’all
Do not = don’t
1979 = ‘79
• Note that the current convention concerning an entire decade does not include an apostrophe: She remembers growing up in the 1960s.
• Some cautions with possessive forms include the following:
Pronouns that do not have an apostrophe but are possessive: his, hers, its (don’t confuse with the contractions “it’s”), ours, yours, theirs.
Distinguish carefully between possessive and plural constructions in sentences such as the following (the distinction is based on your intended meaning in the sentence):
The principal did not approve of the students’ singing in the lunchroom. (The lack of approval concerns the singing, but not the students as people.)
The principal did not approve of the students singing in the lunchroom. (The lack of approval concerns the students, regardless of their singing.)
Be careful with meaning also when you have compound possessives:
Steve and Anne’s house is in our neighborhood.
Mark’s and Ted’s homework assignments were exactly the same, including the incorrect answers.
On-line help can be found here:
http://owl.english.purdue.edu/owl/resource/621/01/
