Apostrophes show possession (ownership), indicate a contraction (the omission of one or more letters or numbers), or are used to form the plurals of some letters and abbreviations.

Possessives

Use an apostrophe to form a possessive noun or pronoun.

  1. When the noun or pronoun is singular, put the apostrophe after the last letter in the noun and then add an s.
    • The dog’s collar is red.
    • Smith’s theory validates these findings
  2. When the noun or pronoun is plural, just add an apostrophe to the end of the word.
    • The dogs’ collars are red.
    • The participants’ responses were anonymous.
  3. Use the apostrophe even when the item in possession is not stated, but is implied.
    • We went to Doug's house for dinner.
    • We went to Doug's for dinner.

Plurals

  1. Lowercase letters and the capital letters A, I, M, and U
    Use an apostrophe and an –s to form the plural of all lowercase letters and the capital letters A, I, M, and U.
    • Be sure to dot your i's and cross your t's.
    • The instructor gave few A's in the class.
    Note: We use an apostrophe to make A, I, M, and U plural because without the apostrophe it would form a different word (A's–As, I's–Is, M's–Ms, and U's–Us).
  2. Words used to refer to themselves
    Use an apostrophe and an –s to form the plural of a word used to refer to the word itself.
    • Make sure you cover all the if's, and's, and but's.
  3. Abbreviation that contains periods
    Use an apostrophe and an –s to form the plural of an abbreviation that contains periods.
    • The university has many Ph.D.'s on its faculty.
    • R.N.'s who wish to further their education will like our evening program.

Other Uses

  1. Forming contractions
    Use an apostrophe to form contractions of the words it is, do not, and who is. (Contractions are used only in informal writing.)
    • It's going to be fine.
    • Don't play that tape.
    • Who's going to the movie 
  2. Omitting letters
    Use an apostrophe to indicate that figures or letters have been omitted on purpose.
    • The report was produced in '95.
    • Here's wishin' you luck.
  3. Abbreviation without periods and symbols
    For numbers, abbreviations without periods, and symbols used as words, the apostrophe before the –s is optional if the plural is clear.
    • 7s OR 7's
    • 1960s OR 1960's
    •  UFOs OR UFO's