The indefinite article "an" is used when the following word starts with a vowel sound. For example one write "an adjective" but "a noun".

However, when an abbreviation is used, should this be based on the expanded meaning, or on the letters of the abbreviation.

Specifically, one writes "a noun phrase". However, here on ell.se, the abbreviation "NP" for 'noun phrase" is commonly used. If this were pronounced by sounding out its letters, as "En-Pee" it would call for "an". If it were read as "noun phrase" it would call for "a". Should one write "a NP" or "an NP" here on ELL?

I incline to "a NP" and i used that in an answer just a few minutes ago, but I am not at all sure.


It depends.

As you've identified, it's usually based on the pronunciation. Acronyms will have a common pronunciation which should be followed; a NATO would be correct vs. an N.A.T.O (en-ay-tee-oh) which would be incorrect. Abbreviated notations are another issue though; in the previous sentence "vs." was used and should be pronounced as "versus", and chemical notations like "Li" are read as "lithium".

In your example, "an" would be more correct. "A" can be used as a means of trying to force readers to sound out the full words, however many people would see it as an error. If you choose to use this method, stay consistent throughout your piece of writing.

  • There is only one NATO, so "a NATO" would be very unusual. Jul 18 '19 at 10:31
  • NATO would be more of a descriptor, like "a NATO military operation".
    – Luck
    Jul 18 '19 at 22:45

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