Style Matters #13: Let’s not be indefinite about when to use ‘a’ or ‘an’


Use of the indefinite article (a or an) has been known to trip up even experienced writers.

Which option to use in front of a particular word, especially a word that begins with a vowel, can sometimes be a little tricky – until you realise that sound and not sight should be your guide.

Let’s work through the main rule and its exceptions.

  • The indefinite article a typically goes before words that begin with consonants (non-vowels): a bag, a cat, a dart, a stick
  • On the other hand, an usually goes before words that begin with a vowel: an apple, an extra bag, an open book

It is the exceptions to this otherwise pretty simple rule where writers typically come unstuck.

The reason for the confusion is that the decision is based on sound (what we hear) rather than what we see (read). So we use:

  • an when a word begins with a silent consonant such as h: an honest mistake, an honourable discharge
  • a when a word (or an acronym) begins with a harsh or sounded consonant such as h: a hotel, a history book, a HDMI cable
  • a when the word’s first letter, a vowel, sounds more like a consonant (such as yu for u, or wa for o) as in: a union, a unit, a united team, a one-legged stool

Common pronunciation should be your guide to which form of the indefinite article is correct.

However, acronyms can also present a few challenges when it comes to pairing them with the correct indefinite article.

With acronyms that are expressed as whole words, writers can usually follow the simple basic rule:

  • a NATO initiative, a JPEG file, a Qantas plane
  • an AIDS counsellor, an EFTPOS machine, an ICAC investigator

However, writers need to take care with acronyms that are sounded out as separate initials, especially when the first initial of the acronym is a vowel or a consonant that has a vowel-like sound when spoken. These are the acronyms that begin with a, e, f, h, i, l, m, n, o, r, s, u or x.

Just as with whole-word acronyms, rely on your ear to help you discern the difference with spelled-out ones:

  • an FAQ, an HMAS ship, an RAN boat, an NRMA roadside service, an MAO drug
  • a BBC news report, a CDC warning, a DFO complex, a WHO alert, a YMCA event
  • but it’s a US fleet or a UN directive  (because we say yu for u in these instances)

Plus there still may be special exceptions in local spoken language usage that will come into play.

In Australia, for instance, we write NSW but will almost always read and say this as New South Wales, so we write/say a NSW court (even though, strictly speaking, the first letter of this acronym is a consonant with a vowel-like sound).

Writers also inadvertently misuse the indefinite article when expressing numerical values, as in:

  • she walked a 100 steps – when read aloud this would be awkward, she walked a one hundred steps
  • instead it should be she walked 100 steps or she walked a hundred steps

Another tip is that good writers avoid using the indefinite article with numbers – e.g., in imprecise phrases such as a number of or a percentage of – by going to the trouble of giving a precise or estimated number.

Finally, one of the most difficult aspects of English language learning for both native and non-native learners is figuring out the correct time to use the indefinite article (a and an) or the definite article (the).

Learners often confuse the two and consider them interchangeable when, in fact, they have quite different roles. To clarify that confusion:

  • use the indefinite article, a or an, to refer to any person, place or thing
  • use the definite article, the, to refer to a specific person, place or thing.


(I’d like to acknowledge the inspiration of an eagle-eyed newspaper copy taster I worked with long ago, Kim Lockwood, one of the skilled contributing authors to earlier editions of News Limited’s in-house Style guide for journalists)


If you like what you’ve read here, you can see reporting4work’s similar posts at Style Matters

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