Apostrophe Usage

In Contractions

Use an apostrophe to replace letters that are omitted in a contraction. (A contraction is a shortened form of a word, in which the position of the missing letters is usually marked by an apostrophe. ) 

can’t, it’s, you’re

In Place of  Numerals

Use an apostrophe to replace omitted numerals. 

Technology of the ’90s

For Possessives

Use an apostrophe to denote the possessive case of a noun. 
  • Add an apostrophe and an “s” to most indefinite pronouns, singular nouns (including collective nouns), and plural nouns that do not end in “s.” 
someone’s system, the group’s privileges, people’s rights
  • To form the possessive of singular nouns ending in “s” or its sound, you often add an apostrophe to the word and then an “s.” 
the mouse’s buttons, the bus’s capacity
  • Add just an apostrophe when the addition of an “s” produces an awkward sound. 
Plirg Systems’ employees

In a few cases, however, either is acceptable.

Please compare: M. Travis’s files and M. Travis’ files
  • Add just an apostrophe to form the possessive of plural nouns that end in “s.” 
users’ passwords
  • Add an apostrophe and an “s” to the last word of a compound to form the possessive of most compound constructions. 
somebody else’s tasks