Indefinite articles

      We all learned the rule that "a" is used before words beginning with a consonant (a dog, a cat) and "an" before words beginning with a vowel (an eye, an oyster). We are taught basic rules like this because they work most of the time. Then we progress to the exceptions, all caused by silent consonants (an hour, an honor, an heirloom) or vowels that sound like consonants (a uniform, a European vacation). In fact, the choice of indefinite article is governed by the sound of the next word, not by its spelling.
      One interesting case is "history" and its derivatives. We have all seen phrases like "an historic document," which would seem to be an exception. In fact, these phrases are incorrect in American English. Since we pronounce "historic" with the same definite "h" as in "history," we must write "a historic document." Why then does "an historic document" live on? I suspect that it derives from British accents that slight the "h" or eliminate it altogether. This in turn probably comes from the French way of dropping the initial "h", for example in "l'homme" (roughly "loam").
      Having said all this, I think that most people get the indefinite article right nearly all the time because they do it by ear, the right way. But there are some interesting tricky points worth mentioning. One has to do with acronyms, whose article depends on whether the letters are spoken as a groups (a true acronym) or pronounced individually. For example, if we refer to a discussion group within NOAA (National Oceanic and Atmospheric Administration), we can write "a NOAA group" (making a word out of NOAA) or "an N-O-A-A group," depending on whether we say NOAA or N-O-A-A. If we want to eliminate the second indefinite article before a pair of words (as in "a dog and cat"), we may do it only if both take the same article. Otherwise, we have to write out the second article (as in "a prize and an honor").

The Indefinite Grammar Guy