The grammar rule most of us learned in grade school was to use a in front of words starting with a consonant, and use an when the word starts with a vowel (i.e., a, e, i, o, and u). Instead, the use of a and an is actually governed by whether or not the word starts with a vowel or consonant sound. While most word’s first letter is generally consistent with the sound of the first syllable (i.e. a consonant with a consonant sound, or a vowel with a vowel sound), that is not always true. As is the case with most grammar rules, this can become a little confusing. For a better understanding, reference the following examples:
Using a when letter and sound are consistent:
- a stapler
- a computer
- a project
- a memo, etc.
Using an when letter and sound are consistent:
- an office
- an automated response
- an inkjet printer
- an unclear question, etc.
Those were easy, right? Here’s where is gets tricky:
Using a when letter and sound are not consistent:
- a unit (because it begins with a y sound)
- a one-time request (because it begins with a w sound)
- a European client (because it begins with the y sound), etc.
Using an when letter and sound are not consistent:
- an honest answer (because the h is silent and the initial sound is an o —vowel— sound)
- an hour (again, because of a silent h and an o sound)
- an MBA degree (because the initial sound is an e sound), etc.
If you pay attention to the way you use these articles (a and an) when you speak, you’ll find it comes pretty naturally to pair a with consonant sounds and an with vowel sounds. Correctly following the rule in writing is a little less intuitive, but sounding out the word will help to ensure proper usage.
For more information, see Grammar Girl’s post on the topic.