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Here is the situation. I am in an office full of physicists and one physicist is writing his PhD Thesis. Fundamentally, he wants to know whether he should type "an SV" or "a SV" into the computer.

SV is an acronym for "Secondary Vertex", so it is clear to us that "a secondary vertex" is correct.

So the argument is over whether

  • a or an "sounds better"

and additionally

  • is SV a new, alternative word which means the same thing as "secondary vertex" (just as one can use "roll", "bap", "barm" interchangeably in the phrase "bacon roll" to mean some bread which is probably also buttered into which one has inserted some grilled, or otherwise cooked using another method, bacon causing the butter to melt into a delicious snack most suited for eating at lunchtime.

  • or is SV an object which is to be expanded by the readed using a previously defined definition (should one read "SV" as "secondary vertex"?)

So, he should type "a secondary vertex", but is it "an SV" or "a SV". My vote is for the former as it "sounds better". (Or at least that is my opinion.)

Finally, we also consider "an STFC" or "an SRVTLVF" to be "better sounding", however "a TRVTLVF" seems to be "better sounding". We have discovered that it doesn't seem to matter what the letters of an acronym following a first "S" letter are, the "better sounding" choice is still "an" rather than "a".

Is there a rule about whether "a" or "an" should be used for acronyms?

marked as duplicate by RegDwigнt Aug 9 '17 at 11:09

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  • 3
    Whether or not "SV" is pronounced "ess-vee" or "secondary vertex" is entirely up to the writer. If the writer pronounces it "ess-vee", he will write "an SV" as it begins with a vowel sound. If the writer pronounces it "secondary vertex", he will write "a SV" as it begins with a consonant sound. The reader then, in turn, can deduce the writer's pronunciation from the article. If I see "an SV", I know for a fact I am supposed to read it as "an ess-vee" and not as "an secondary vertex", because the latter is not English. – RegDwigнt Aug 9 '17 at 11:14
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    Generally speaking, unlike any other rule of English, this one rule is a) insultingly simple, and b) holds exactly 100% of the time with no exceptions whatsoever at all. Before a vowel sound, use "an". Otherwise, use "a". Sound, not letter. Thus, "an apple", "a pear", "an SUV", "a year", "a why", "a one". – RegDwigнt Aug 9 '17 at 11:21
  • @RegDwigнt - "Holds exactly 100% of the time with no exceptions whatsoever at all"? Barrie England would disagree with you... – AndyT Aug 9 '17 at 11:36
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    @AndyT You're confusing sounds with spellings. There are no exceptions to the sound law. – tchrist Aug 9 '17 at 11:46

The choice of "an" or "a" with abbreviations is determined by their sound. Imagine spelling them phonetically, and then using the usual rules.

So, we say "a secondary vertex" or "an SV". ("An ess-vee", thinking of it phonetically). As you say, it sounds better, and more importantly it's easier to say. Saying "a SV" is awkward, "an SV" is much easier. Following this rule you would say "a TRVTLVF", because spelled phonetically, "TRVTLVF" starts with a consonant not a vowel.

It doesn't matter that it changes between the case where you use the full name and the case when you use the abbreviation, because they are different words. That is, even if "SV" means the same thing as "secondary vertex", it is still a different word, and so can have a different indefinite article.

Also, as a side note, it's only called an acronym if you pronounce it as a single word. If you spell out the letters it's an initialism. Both are abbreviations. So, NASA is an acronym because we say "nah-sah", not "N-A-S-A". NSA is an initialism because we say "N-S-A", not "un-sah" or something. So, STFC, SV, SRVTLVF, etc, are all initialisms. Many people don't know what an initialism is so it's often simpler to refer to them all as abbreviations.


The usage of the indefinite article a or an is determined by the sound following it, not the vowel letters. If the abbreviation is preceded by an considering its initial vowel sound, it should be read so as to get that vowel sound. an SV should be read as an essvee. The same way an Msc student cannot be read as an Master of Science student, it should be read as an Msc student.

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