Using An and A (especially with Abbreviations and Acronyms)

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.
Got it? Take a quick test.

Using A and An

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.

Examples:
  • 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, it is about the sound of the first letter.)
  • 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.

More examples:
  • 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 best 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.
Interactive Test
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See Also

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