We were taught in school to use "a" before words that start with a consonant and "an" before words that start with vowels. Yet, I don't recall anyone ever telling me which article to use before an acronym ― and not just any acronym, but specifically those that start with A, E, I, O, U, F, H, L, M, R, S, and X and for which each letter is pronounced separately.
To be clear about this last point, sometimes an acronym spells out a word. For example, Mountain Equipment Co-op's acronym MEC is pronounced "meck."
In the above sentence, MIT is pronounced em-ai-tee. The acronym itself starts with a consonant, but the pronunciation begins with a vowel: "e" for "em."
Now consider "ef" (F), "aetch" (H), "el" (L), "ar" (R), "es" (S), and "ecks" (X). Any acronym that starts with one of these and pronounces each letter separately (also called initialism) must be preceded by "an" if an article is to be used.
Just a quick revisit to articles: "the" is a definite article and "an" and "a" are indefinite articles.
So, if you're going to use an indefinite article before an acronym beginning with A, E, I, O, U, F, H, L, M, R, S, or X, stop and think twice ― especially if you're making a sign or a business card, or something will take a lot of money to undo.
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Meet the Editor
I'm Coreen, and I am a copy editor, writer, instructor, digital marketer, and student of PR and Communications for organizations doing positive work in the world.