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GSC Guide to Authors
The indefinite articles 'a' and 'an'

 Grammar
  1. Introduction
  2. Sentences
  3. Jargon and Contrived or Redundant Words
  4. Compounding of Words
  5. Numerical Expressions
  6. Parallel Structures
  7. Nouns
  8. Pronouns
  9. Verbs
  10. Adverbs
  11. Adjectives
  12. Conjunctions
  13. The Indefinite Articles 'a' and 'an'
  14. Prepositions
  15. Capitalization
  16. Italics
  17. Verbosity
  18. Selected Bibliography

The indefinite article a is used when the word following begins with a consonant (i.e. a letter of the alphabet other than a vowel) or a consonant sound. An is used when the following word begins with a vowel (i.e. the letters a, e, i, o, u). The form of the indefinite article fluctuates, however, before some words beginning with h, depending upon the pronunciation of the h; this includes the aspirated h (with an audible h-sound), and the y- and w-sound heard in union and one.

a horst, a history, a habit, a halide, a hand level, a halo, a hand specimen, a hanging wall, a hardpan, a heavy liquid, a hinge, a hill, a hiatus, and a hoodoo.

Some words are spelled with an initial vowel, but pronounced as a consonant: a European fossil, a euphemism, etc. Conversely, words spelled with an initial consonant, but pronounced with a vowel, take an: an MP.

The article an is used for a before vowels and before an unaspirated h:

an elevation
an IUGS scientist
an n-n log
an hour
an M.A.
an oolite
an honours degree

2005-11-21
http://ess.nrcan.gc.ca/pubs/scipub/guide/gramm/artic_e.php