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For choosing a or an, spelling doesn't matter; pronunciation does. A is for consonant sounds; an is for vowel sounds. The ever-popular an historic is incorrect, at least for American speakers, because historic does not begin with a vowel sound. Even those Americans who say "an istoric" will admit that they say "historic," with the consonant h, when the word stands alone. I don't care whether "an istoric" rolls off your tongue more easily than "a historic"; you don't go altering your pronunciation of a word in order to change the article you use before it. Your comfort is none of the language's concern.

Most of the times I've heard "an historic," however, it has been from blustery types who heartily pronounce the h. Think Howard Cosell.