Taking notes, I was going to write this:

therefore p′ is a shortest path.

But the topic is APSP which stands for "all pairs shortest paths" so I decided to just write SP instead and...

Is it written with a like this

therefore p′ is a SP

or with an like this (because in my head I would "read" the S)

therefore p′ is an SP

  • Maybe this is a jargon thing, but shortest means the one with the least length. There can only be one such shortest path in most cases, unless another is identical, in which case you'd normally say "one of the two shortest paths". Commented Jan 9, 2018 at 14:50
  • 5
    @JimMacKenzie: When you want to fly from the north pole to the south pole, there are infinitely many different shortest paths that you could take. Situations like these are quite common occurrences, especially in science and technology, and so talking about a multitude of shortest paths (or largest objects, most cromulent words, ...) is a perfectly normal thing to do in these contexts.
    – RQM
    Commented Jan 10, 2018 at 11:48
  • You might also consider that whether you use “a” or “an” would encourage people to pronounce “shortest path” or “ess pee” respectively. Commented Jan 10, 2018 at 12:55

4 Answers 4


Whether you say a or an is determined by the pronunciation of the next word, and nothing else. If the next word begins with a consonant sound (not necessarily a consonant letter!), you say a, and if it begins with a vowel sound (not necessarily a vowel letter!), you say an.

So yes, you are correct: if you write or read "SP", you would say "an SP", because "ess" begins with a vowel sound.

  • 17
    I don't think the answer is this simple, although I admit I don't have the answer. When you read some abbreviations, you pronounce them as initialisms in your head. Others, you don't. For example, Some people will read "45 mpg" out loud as "forty five miles per gallon," and others as "forty-five em-pee-gee." Some will read "59 m" as "59 meters." I don't know of anyone that would read it out loud as "59 em." The same can be said of reading "SP." Some people will read it out loud as "ess pee" and others might read it out loud as "Shortest path." Incidentally, I'm in the latter category. Commented Jan 8, 2018 at 17:42
  • 10
    @RandallStewart Well, yeah: the pronunciation of the next word, whether you pronounce it as "ess pee" or "shortest path", but if you write "an SP doodad", that's a pretty big clue you intended it to be read as "an ess pee doodad". I'm trying to think of an example in which you would put an article directly before a something you don't spell out, but I'm not coming up with one.
    – stangdon
    Commented Jan 8, 2018 at 17:52
  • 3
    But that's only an issue if you write "a/an SP". If you write "a shortest path", then there's no question that "shortest path" is pronounced "shortest path". To say that someone might read it "SP in his head ... by that reasoning, if I write "a state in the west" I should use "an" because someone reading that might think of Idaho. :-)
    – Jay
    Commented Jan 8, 2018 at 17:58
  • 9
    An example for your comment would be "an FBI agent" as I don't think someone would read that as "a Federal Bureau of Investigation agent". I think people tend to read abbreviations as letters if the abbreviated text is long, but there is a thick line between what's short and what's long (an example would be "shortest path", we can't tell for sure if it's going to be read "ess pee" or "shortest path"). Commented Jan 8, 2018 at 22:12
  • 7
    Also note that the writer's choice to use a/an often expresses an intent to the reader for how the abbreviation/acronym should be read. Commented Jan 9, 2018 at 18:51

These are actually called acronyms or initialisms. The few editing guides I checked (like this one from the American Psychological Association) say to use this guide:

The general rule for indefinite articles [before acronyms] is to use a before consonants and an before vowels. The trick here is to use your ears (how the acronym is pronounced), not your eyes (how it's spelled).

This means you have to "sound out" the word in your head to tell if it starts with a consonant sound or a vowel sound. Examples:

I checked this answer with an APA editor. (ayy pee ayy)

Today an FBI agent came to our office. (eff bee eye)

Today a CNN reporter asked me questions (cee enn enn)

Today an NBC reporter asked me questions (enn bee cee)

However, this depends on whether you commonly sound out the individual letters, or if the acronym is usually pronounced as a word. For example, a LAN (computer network) is usually pronounced as it's spelled, so you would say:

a LAN schematic

Similarly SAM, SIM, SCUBA, GIF, JPEG, ZIP, LASER, IMAX, and others.

Other examples:

  • a FASB rule; an FOB airfield
  • a LAN schematic; an LAPD memo
  • a MOMA exhibit; an MRI test
  • a NICU nurse; an NPO order
  • a SAM base; an SAT exam

(Edit) To add further complication: As choster mentions below, not everyone pronounces all of these kind of abbreviations the same way. For example, with SQL, some people say "ess cue ell" and others say "sequel".

In the first case you would say, "an SQL query" and the second, "a SQL query"

  • 18
    APA, FBI, CNN, NBC are not acronyms, which are pronounced as words. They are merely garden-variety abbreviations, or, if you prefer, initialisms. (See the Can be confused and Grammar note sections at any of those links.) OTOH, I have heard NICU pronounced "NICK-you" (ˈnɪk-ju), and I think I have heard MOMA pronounced ˈmoʊ-mə.
    – shoover
    Commented Jan 8, 2018 at 18:24
  • 3
    @shoover good to know, but your own link contradicts you with the second definition. Initialisms, while accurate, isn't common parlance.
    – Andrew
    Commented Jan 8, 2018 at 18:47
  • 2
    I can confirm that "nick-you" is what all the doctors and nurses say at least in the US.
    – mattdm
    Commented Jan 8, 2018 at 22:04
  • 3
    A FOB (forward operating base) is, at least in the US military, pronounced fob, not via initials. Commented Jan 8, 2018 at 22:44
  • 1
    A database program I once developed for offered both an SQL and a SEQUEL interface. They were similar but distinct query languages. (As I recall SEQUEL was a follow-on to QUEL.) The database vendor's trainers and support folks were very sensitive about carefully distinguishing between them. Commented Jan 9, 2018 at 19:22

I've left a comment already, and the OP has already selected the accepted answer, but I feel compelled to leave a full answer of my own because it feels like there are a number of loose ends.

First, it's well established that the use of a vs. an is determined by the sound that comes next, not necessarily the written letter. The Chicago Manual of Style (16th edition) puts it like this:

"A comes before words with a consonant sound...no matter how the word is spelled {a eulogy}"

But the OP seems to understand this already, based on the wording of his text. The title of this question has to do specifically with abbreviations. Here things get tricky. You should still chose the article adjective based on the sound it precedes. But when an abbreviation can be pronounced more than one way, and the pronunciation determines the article, how do you pick the correct article?

Some abbreviations are almost always pronounced as the full word or phrase that they represent:

  • "A Mrs. Smith called you right after you left." (Not: "An Emm-Are-Ess called you...")
  • "Please bring a no. 2 pencil." (Not: "Please bring an En-Oh two pencil.")
  • "We traveled in a NW direction." (Not: "We traveled in an En-Double-Yoo direction.")
  • "A He-filled balloon (Not: "An aitch-ee filled balloon.")

Some abbreviations (intitialisms) tend to be pronounced letter by letter, while others (acronyms) tend to be prounounced as single words:

  • An FBI investigation, An AT&T investment.
  • A Radar detector, a Scuba instructor

That leaves the 3rd class of abbreviations: Their pronunciation hasn't settled into a standard form yet. I think this is the part of the OP's question that the other responses haven't addressed and that feels like a loose end.

  • NICU: This is sometimes pronounced En-Eye-See-Yoo or Nick-Yoo. (Example from Shoover above.)
  • SQL: Pronounced as an initialism or an acronym (Example from Andrew above).
  • ROTC (Reserve Officer Training Corp): Pronounced as "rot-see" or "are-owe-tea-sea" (example from Todd Wilcox above)
  • FAQ (Frequently Asked Questions): Pronounced as "fack" or "eff-ay-cue" (example from TripeHoward above)

So in print: For abbreviations without a traditional, well-understood pronunciation, how do you choose the correct article adjective? How do you know if an abbreviation is meant to be read out loud as:

  • the full word?
  • an initialism?
  • an acronym?

The use of all caps is no help. For example, if read out loud, don't you pronounce TBSP as "Tablespoon" and BLVD as "Boulevard"? I've seen Stack Exchanged abbreviated as "S.E.," but don't you still read that aloud as Stack Exchange?

I would put the OP's "SP" in this category since it's a non-standard abbreviation that the OP came up with him/herself. I can think of three options to pick the right article:

  • Mention early in the paper how the authors pronounce the abbreviation, and then use the appropriate article adjective. (This feels stodgy, but it could be helpful for an uncommon abbreviation that, nonetheless, still has a standard pronunciation in a certain field or context.)
  • Reword the sentence to avoid the ambiguity. (This is a good old fallback, but it sort of avoids the question rather than answer it.)
  • The Chicago Manual of Style has this to say:

"Before an abbreviation, a numeral, or a symbol, the use of a or an depends on (or conversely determines) how the term is pronounced." [emphasis mine]

In other words, you can choose whichever article adjective you prefer, and that should guide the reader how you intend the abbreviation to be pronounced.

  • 2
    TBSP: Nope; didn't realize it was an all-caps version of Tbsp. BLVD - can't tell (I scanned Boulevard at the same time as BLVD, but probably "no"). I might give a different answer if there was a bit more context to prompt me (a discussion of recipes or street addresses). On the other hand I definitely read "S.E." as "ess ee", not "stack exchange". Note that Tbsp and Blvd are different to SE and SP. The former pair are abbreviations of a single word, and the latter pair are abbreviations of phrases from their initial letters (which is why I find TBSP and BLVD wrong). Commented Jan 10, 2018 at 8:14

The general rule is that when the initial letter of a word has a consonant sound, article "a" takes precedence and when it's a vowel sound, article "an" is used. This however depends on the pronunciation of certain words inform of initials, acronyms or abbreviations.

  • Yes, but the question is tangled between the abbreviation and the vowels and the non definite article.
    – user17814
    Commented Jan 10, 2018 at 17:38

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