A and an are the two forms of the English indefinite article. They mean the same thing — roughly "one" or "some unspecified one" — and they're used before a singular countable noun that the listener hasn't heard about yet.
The choice between them is purely mechanical. It has nothing to do with meaning and nothing to do with the letter the next word starts with. It depends on the sound.
The rule in one line
Use a before a consonant sound. Use an before a vowel sound.
| SOUND | ARTICLE | EXAMPLE |
|---|---|---|
| consonant | a | a book, a car, a teacher |
| vowel | an | an apple, an egg, an idea |
That's the whole rule. Everything that follows is about the one word students always trip on: sound.
Sound, not letter
English spelling and English pronunciation often disagree. The article follows what you hear, not what you see.
A word can start with a vowel letter but a consonant sound:
University, year, one, and European all begin with a y or w sound. Those are consonant sounds, so the article is a.
A word can also start with a consonant letter but a vowel sound. The most common case is a silent h:
The h in hour, honest, and honour is silent, so the word actually starts with a vowel sound. Compare it with house or hotel, where the h is clearly pronounced: a house, a hotel.
Acronyms and initialisms
The same rule applies to acronyms — you go by how the letters are said out loud.
When the first letter is pronounced as a name (em, ef, el, en, ar, es), it starts with a vowel sound, so the article is an, even though the letter itself is a consonant.
When the acronym is read as a single word (NATO, NASA), or when the first letter name starts with a consonant sound (U as yoo), the article is a.
MBA begins with em (vowel sound → an). UN begins with yoo (consonant sound → a). NATO is read as one word starting with n (consonant sound → a).
When a and an are used
Beyond the sound rule, the article itself signals three related ideas. All three are part of the same basic meaning: "one of many, nothing specific."
First mention. You introduce something the listener doesn't know yet. The next time the same thing comes up, it becomes the.
One of a group. The noun could be any member of a larger class — it doesn't matter which one.
Meaning "one". A and an can replace the number one in quantities and measurements.
A and an are also standard before job titles, nationalities, and descriptions of people. She is a doctor. He is an architect. Sarah is a Canadian. These belong to the "one of a group" idea — she is one doctor among many.
A and one
Both a/an and one can introduce a single unspecified noun — but they are not interchangeable. The difference is emphasis.
a/an is neutral. It introduces the noun as part of the normal flow of the sentence, without drawing attention to the count.
one puts the spotlight on the number — one, not more. Use it when the count itself is the point: in contrasts, precise requests, and measurements where the number carries meaning.
The same pattern works for round numbers. A hundred, a thousand, a million are the casual versions; one hundred, one thousand, one million feel slightly more formal or more precise. Both are correct.
When in doubt, use a/an. Reach for one only when the number carries meaning of its own.
Adjectives change the sound
When there is an adjective between the article and the noun, the article follows the adjective's first sound, not the noun's.
The noun doesn't matter here. What matters is the word that comes right after the article.
Common mistakes
Sound, not letter
Acronyms
Adjective in the middle
Missing article with a job or role