Grammar/Using the indefinite articles - ‘A’ and ‘An’

Articles: Using the indefinite articles - ‘A’ and ‘An’

Writers in STEM fields use the indefinite articles, ‘a’ or ‘an’, to refer to something non-specific (or ‘indefinite’) in their writing.

You should speak a word rather than read it to help you decide whether to use the indefinite article ‘a’ or ‘an’; although there are some exceptions, you should generally use ‘a’ when referring to a word that makes a consonant sound, and use ‘an’ when referring to a word that makes a vowel sound.

For example, you should write: “A rabbit…” or: “A giraffe…” because these words begin with consonants (and make consonant sounds when spoken). However, you should write: “An elephant…” or: “An anteater…” because these words begin with vowels and make vowel sounds when spoken).

The reason that it is helpful to speak words aloud when deciding whether to use ‘a’ or ‘an’ is because ‘silent letters’ could otherwise confuse you when simply seeing them written.

For example, you should write that: “Professor Reilly scored a hat-trick,” (because the ‘h’ in this word makes a consonant sound), but you should write: “The same player acted in an honourable way when passing up another goal due to an opposition player being injured,” (because the ‘h’ in this word is silent, which means the ‘o’ is the first letter you hear, and this ‘o’ makes a vowel sound).

This same general rule applies when using acronyms in your writing, which is why you should write: “A NASA spacecraft is currently taking pictures of Mars,” but: “An EPA directive ensures that businesses attempt to reduce their carbon emissions.”