When to Use "A" vs "An"

Use "an" before a word that starts with a vowel SOUND, and "a" before a word that starts with a consonant SOUND — this is about pronunciation, not spelling. Most of the time that lines up with the letter itself (a dog, an apple), which is why people learn it as a spelling rule — but the real rule is about sound, and that's what explains every exception.

The basic version

• A before a consonant sound: a car, a house, a plan
• An before a vowel sound: an idea, an egg, an hour

Where it gets interesting: silent letters and sneaky sounds

Because the rule is about sound, not the letter on the page, some words break the "obvious" pattern:

• "An hour" — the H is silent, so the word actually starts with a vowel sound ("our"). Same with "an honest mistake", "an heir".
• "A university" — starts with the letter U, but it's pronounced "yoo-niversity", and Y is a consonant sound. So it takes "a", not "an". Same with "a European trip", "a one-off" (sounds like "won").
• "An MBA" — M is a consonant letter, but said aloud "MBA" starts with "em", a vowel sound. So it takes "an". Same logic applies to other letters read by their names: an FAQ, an SOS, an X-ray.

Quick test

Say the next word out loud (or say the acronym letter-by-letter if it's an abbreviation). If the very first sound is a vowel sound (a, e, i, o, u sound) — use "an". If it's a consonant sound — use "a", even if the spelling looks like it should be the other way.

More examples

• "a historic event" (H is pronounced — consonant sound)
• "an honour" (H is silent — vowel sound)
• "a unicorn" (sounds like "yoo" — consonant sound)
• "an umbrella" (genuine vowel sound)