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If I use an abbreviation like MMU ("memory management unit" in computers), should I write "A MMU" or "An MMU"? Is it the pronunciation or the letter that decides?

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The thing that matters is whether or not the proceeding sound is a vowel sound. There's actually a very similar example on the Grammar Girl site, A Versus An (emphasis mine):

The letters o and m can be tricky too. Usually you put an before words that start with o, but sometimes you use a. For example, you would use a if you were to say, “She has a one-track mind,” because one-track starts with a w sound. Similarly, “She has an MBA, but chooses to work as a missionary.”

Assuming you pronounce it em-em-yoo (initialism), then I would use "an MMU".

  • I would only add that if the abbreviation were pronounced as a word, you would use 'a'. For example, we refer to "a MIMO channel" because "MIMO" is usually pronounced "ma-i-mo" and not "em-ay-em-o". Article choice depends on pronunciation and not spelling. – urnonav Sep 7 '17 at 2:15

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