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a, an

Use a before a consonant sound:
            a historical guide, a onetime king, a U-turn.

Use an before a vowel sound:
            an onion, an 11-year-old child, an L-shaped structure.

Before abbreviations, including acronyms, use a or an according to the pronunciation:
            an AWOL soldier, a GI.

Words with a voiced h are preceded by a (e.g., a historical guide); an unvoiced h takes an:
            heir, herb, honest, honor, hour.

Most of the derivatives of these five also take an except for the derivatives of herb, most of which take a:
            an herb, a herbivore.

After a possessive, omit initial A or An of a title:
            Dickens's Tale of Two Cities, Ambrose Bierce's Occurrence at Owl Creek Bridge.