 The form "an" is used before words that begin with a vowel sound (even if spelled with an initial consonant, as in "an hour"), and "a" before words that begin with a consonant sound (even if spelled with a vowel, as in "a European").Before some words beginning with a pronounced (not silent) "h" in an unstressed first syllable, such as "historic(al)", "hallucination", "hilarious", "horrendous", and "horrific", some (especially older) British writers prefer to use "an" over "a" ("an historical event", etc.).