eli5 Why, in English Grammer, is it always an “an” before an acronym? Even if the acronym doesn’t start with a vowel?

Question

eli5 Why, in English Grammer, is it always an “an” before an acronym? Even if the acronym doesn’t start with a vowel?

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Answer 1

It’s not always, but often. And it has to do with the sound of the acronym, or initialism, rather than the letter it begins with.

So we might say “a CPU,” because phonetically, CPU starts with an “s” sound (see-pee-you). “An CPU” would sound wrong to our ears.

But we would say “an FBI agent”, because FBI phonetically starts with a short “e” sound (eff-bee-I.” We change the article based on sound, not on spelling.

It’s not an issue that comes up with most words, because if they start with a consonant, they typically have a consonant sound. But a lot of letters, when spoken, begin with a vowel sound, so we accommodate that.