a or an? The rule is that an should be used instead of a in front of all words beginning with a vowel sound. These include all words whose first letter is a or i, most words beginning with e, o, and u, and a few beginning with h: an ace; an interesting evening. The exceptions among words spelt with an initial vowel are as follows. When u is pronounced yoo, it is preceded by a: a unit; a useful tool. Words beginning with eu (also pronounced yoo) take a: a eucalyptus tree; a European. One and words beginning with the same sound take a: a one-off; a once mighty empire. Words spelt with an initial h that is not pronounced take an: an hour; an honour. Most modern authorities recommend that other words beginning with h should be preceded by a: a hacker, a horse, a historian, a hotel. It is not strictly incorrect, however, to write an hotel or an heroic achievement, and in speaking it may seem less awkward to say an (h)otel than a hotel