A and an are different forms of the same word, the indefinite article that often precedes a noun.

• A is used before a noun that starts with a consonant sound (e.g., “s,” “t,” “v”).

• An is used before a noun that starts with a vowel sound (e.g., “a,” “o,” “i”).

Note that the rule is not whether they start with a consonant or vowel, but whether they start with a consonant or vowel sound. This can help you decide which to use in difficult cases like words beginning in “u” or “h.”

Note

The indefinite article is used to refer to a non-specific noun. However, the form the indefinite article takes depends on the initial sound of the word directly following it, regardless of whether this is a noun or an adjective.

• An apple

• An delicious apple

• A delicious apple

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A or an before H

The rule of thumb for words beginning with “h” is to consider the way the word is pronounced. Words that have a silent “h” begin with a vowel sound, so they use “an.”

• For words where the “h” sound is pronounced, such as hat, hotel, or hard, use “a.”

• For words where the “h” is silent, such as honor, hour, or honest, use “an.”

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Example: Words beginning with “h”

She wore a hat to keep out the sudden chill in the air.

They bought a house in May of last year.

He was thought to be an honest person, so his lies came as a surprise.

The meeting took an hour.

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A or an before U

Similarly, for words beginning with “u,” consider the way the word sounds.

• For words where the “u” sound is pronounced like “you,” such as user, usual, or utilized, use “a.”

• For words where the “u” sound is pronounced like “uh,” such as unusual, understanding, or utter, use “an.”

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Example: Words beginning with “u”

The email was about an urgent matter.

They came to an understanding about the cost of the repair.

It would have been helpful to have a user manual.

The invention of scissors proved a useful one.

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A or an before acronyms

The same rule applies to acronyms—say the acronym aloud to sound out whether it begins with a consonant sound or a vowel sound.

Some acronyms (like “NASA” [nass-uh]) are usually pronounced as full words, others (like “TV” [tee-vee]) by saying the individual letters. But remember that even the spoken form of a consonant can begin with a vowel sound (e.g., [em] for M, [aitch] for H).

• Acronyms beginning with a consonant sound use “a.”

• Acronyms beginning with a vowel sound use “an.”

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Example: Acronyms

A NATO [nay-toe] regiment landed amphibiously during the night.

The hospital acted swiftly to prevent a HIPAA [hip-uh] violation.

An MRI [em-arr-eye] machine uses magnets to take detailed scans of your organs.

It can be expensive to pay for an SAT [ess-ay-tee] prep course out of pocket.

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Worksheet: A vs. an

Test your knowledge of the difference between “a” and “an” with these practice sentences. Fill in either “a” or “an” in each sentence.

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1. The trail ended at __ hut.

2. I was not __ witness to anything, I promise!

3. He accidentally ate __ poisonous mushroom.

4. The waiter brought us __ apple pie.

5. __ HIV-positive diagnosis is typically treated with antiretroviral therapy.

6. She has been in there for __ hour already.

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Frequently asked questions