Using "An" and "A" (especially with Abbreviations and Acronyms)

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The Quick Answer
Use "an" before a word that starts with a vowel sound. If it does not start with a vowel sound, use "a." For example:
  • A man
  • An elephant
But, look at this:
  • A house
  • An hour
The key word here is sound. It is not a question of whether the word starts with a vowel. It is a question of whether it starts with a vowel sound.
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using an or a (especially with abbreviations and acronyms)

Using "An" and "A"

There is sometimes confusion about whether to use "an" or "a," particularly with abbreviations. (The words "an" and "a" are known as articles.)

The sound of a word's first letter determines which to use. If the word starts with a vowel sound, you should use "an." If it starts with a consonant sound, you should use "a." For example:
  • Buy a house in an hour.
    • (Although "house" and "hour" start with the same three letters ("hou"), one attracts "a" and the other "an.")
  • An unknown goblin killed a unicorn.
    • (Although "unknown" and "unicorn" start with the same two letters ("un"), one attracts "an" and the other "a." Remember that it is the sound of the first letter that determines whether to use "an" or "a.")
Here are some examples with abbreviations and acronyms (i.e., abbreviations spoken like words).
  • An LRS
    • (LRS = Linear Recursive Sequence)
  • A TT race
    • (TT = Tourist Trophy)
  • It would be a honour.
    • (The word "honour" starts with an "o" sound.)
  • Send an US ambassador.
    • (The abbreviation "US" starts with a "y" sound.)
  • She was involved in a RTA.
    • (RTA = Road Traffic Accident)
    (The abbreviation "RTA" starts with an "a" sound (i.e., "ar-tee-ay.")

Beware of Abbreviations

Abbreviations that start with the consonants F, H, L, M, N, R, S, and X attract "an" because they start with vowel sounds. For example:
  • An FRS representative will be present after lunch.
    • (FRS = Fellow of the Royal Society)
  • A LF transmitter was found in the basement.
    • (LF = Low Frequency)

Beware the Letter U

Abbreviations that start with the vowel U attract "a" because U starts with the consonant sound "y." For example:
  • A US ship spotted a U-boat.
  • An UFO landed in 1967.

Treat Acronyms Like Words Not Abbreviations

An acronym is an abbreviation that is spoken like a word (e.g., BUPA, FOD, FEDEX). Therefore, as the first sound of FEDEX is "f," use "a" and not "an." For example:
  • Tim worked in the air industry as a FOD inspector for a year.
    • (FOD = Foreign Object Damage)
  • Jack was a FEDEX courier.
Read more about acronyms.

An Historical or A Historical?

Letters and sounds do not always correlate in English.

When pronouncing the words "historic" and "historical," the accent falls on the second syllable, and many pronounce them as starting with a vowel. For those people, it is appropriate to use "an" before "historic" and "historical." Therefore, you have a choice depending on what sounds better for you.

There is a lot of leniency on this issue. If you're still unsure, opt for "a historical" and "a historic" as these remain preferable – especially in formal writing.

Read about the difference between "historic" and "historical."
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See Also

What are vowels? What are consonants? What are articles? What are adjectives? What are abbreviations? What are acronyms?