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With initialisms, should one choose "a" or "an" based on the sound of the first letter of the initialism or based on what the first letter denotes?

For example, if "MOU" denotes Memorandum of Understanding, should one choose "a" or "an" based on "em" or based on "memorandum" -- a MOU or an MOU?

The choice presents an issue when the first letter of an initialism is a consonant with a vowel sound or a vowel with a consonant sound, e.g.:

  • FAQ – Frequently Asked Question; a FAQ or an FAQ?
  • LOC – Level of Concern; a LOC or an LOC?
  • MOU – Memorandum of Understanding; a MOU or an MOU?
  • NPV – Net Present Value; a NPV or an NPV?
  • R&D project – Research and Development project; a R&D project or an R&D project?
  • SOP – Standard Operating Procedure; a SOP or an SOP?
  • XRD – X-Ray Device; a XRD or an XRD?
  • URL – Uniform Resource Locator; a URL or an URL?

Also, does the choice depend on whether the writing is formal (e.g., technical, legal, policy) or informal?

marked as duplicate by Janus Bahs Jacquet, Community Aug 19 '16 at 10:01

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • 1
    If you say NPV as /ɛnpivi/ then it's an NPV, because /ɛ/ is a vowel. If you say URL as /juarɛl/ then it's a URL, because /j/ is a consonant. It does not depend on writing at all, and it does not depend on formal or informal. It's only the sound. The pronunciation is the only thing that counts. Not the spelling. Not the letters. The pronunciation. The sound.. A before a consonant sound, and an before a vowel sound. – John Lawler Aug 19 '16 at 4:24
  • FAQ is sometimes pronounced "fack" so belongs with MAP. – Chris H Aug 19 '16 at 6:43

It follows the same rules as the related question you linked. The top answer was:

[...] The question of "a" vs "an" is always decided by the pronunciation of the word that follows the article. Thus, various geographical regions that have different pronunciation rules may use a different article for the same word. [...]

So, apply that rule against the initialisms :

  • LOC starts with a vowel sound (from the 'el' in 'el oh see') -> an LOC
  • MOU starts with a vowel sound (from the 'em') -> an MOU
  • etc etc - all your examples should be 'an', except for URL ('yu').

FAQ is arguable, depending on whether you consider it an initialism ('eff ay kew') or an acronym ('fak').


My common sense says the same rule shall apply here too- with regard to an acronym as it were to apply if acronym was written in expanded form. For example, it shall be a SOP, a SOS and an ISI or an EOU.

  • 2
    Your common sense is at odds with general usage, unless SOP and SOS are pronounced as single syllables (neither of which I've heard). – Chris H Aug 19 '16 at 6:44
  • The answers to the question linked as a duplicate go into more detail than I could in a comment. If they don't explain it to you, perhaps asking a new, specific, question would be a good idea – Chris H Aug 20 '16 at 11:21

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