Which word should be preceded by an?
Just remember that 'ahistorical' means something else entirely. The rule that many people vaguely remember is that one uses a if the word that follows it begins with a consonant, and one uses an if the following word begins with a vowel. Which word use A or an? A and an are two different forms of the same word: the indefinite article a that is used before noun phrases. Use a when the noun or adjective that comes next begins with a consonant sound.