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Articles: A versus An

This resource was written by Purdue OWL.
Last full revision by Chris Berry.
Last edited by Allen Brizee on January 22nd 2008 at 3:27PM

Summary: This short handout deals with which article to use before a noun -- "a" or "an."

Articles: A versus An

How do you know when to use the indefinite articles?

"A" goes before all words that begin with consonants.

  • a cat
  • a dog
  • a purple onion
  • a buffalo
  • a big apple

With one exception: Use "an" before unsounded h.

  • an honorable peace
  • an honest error

"An" goes before all words that begin with vowels:

  • an apricot
  • an egg
  • an Indian
  • an orbit
  • an uprising

With two exceptions: When u makes the same sound as the y in you, or o makes the same sound as w in won, then a is used.

  • a union
  • a united front
  • a unicorn
  • a used napkin
  • a U.S. ship
  • a one-legged man

Note: The choice of article is actually based upon the phonetic (sound) quality of the first letter in a word, not on the orthographic (written) representation of the letter. If the first letter makes a vowel-type sound, you use "an"; if the first letter would make a consonant-type sound, you use "a." So, if you consider the rule from a phonetic perspective, there aren't any exceptions. Since the 'h' hasn't any phonetic representation, no audible sound, in the first exception, the sound that follows the article is a vowel; consequently, 'an' is used. In the second exception, the word-initial 'y' sound (unicorn) is actually a glide [j] phonetically, which has consonantal properties; consequently, it is treated as a consonant, requiring 'a'.

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