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The decision to use a or an depends on the sound—not the spelling—of the word or abbreviation that follows.
A is used before words that begin with a consonant sound. (In their written form, these words may begin either with a consonant or with a vowel that makes a consonant sound, such as eu, ou, or long u.)
An is used before words that begin with a vowel sound. (In their written form, these words may begin either with a vowel or with a silent h.)
The rule stated above for words also applies to acronyms, which are abbreviations pronounced as words:
In the case of initialisms, which are abbreviations pronounced letter by letter, you must go by the pronunciation of the first letter.
If the first letter begins with a consonant sound when pronounced, then choose the article a:
But if the first letter in the initialism starts with a vowel sound when pronounced, then choose the article an:
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