Wendalyn Nichols, editor of the Copyediting newsletter, offers useful tips to copy editors and anyone else who prizes clear and orderly writing. Here she looks at why a seemingly simple rule of English, whether to use a or an as an indefinite article, can cause confusion.

When we are taught how to use the indefinite articles (a and an) in writing, many of us are told that a goes before a word that begins with a consonant and an goes before a word that begins with a vowel.

A single, crucial word is missing from that rule of thumb: sound. Some words that begin with a letter that usually symbolizes a vowel nevertheless do not begin with a vowel sound, and only those that do begin with a vowel sound are preceded by an. If teachers remembered to add (or emphasize) the component of sound, perhaps fewer people would be confused by the seemingly inconsistent treatment of words that begin with the same letter but not the same phoneme.

As it is, however, I seem to get three or four queries about this point every year, and last week came the fourth of 2009, in reference to usual and unusual. Why, the writer asked me, did we use a with usual and an with unusual? After all, the words both start with the letter u.

The answer is in the tail end of the list of vowels you memorized as a child: "A, E, I, O, U, and sometimes Y and W." Not "always Y and W." If you say the word usual aloud, you'll hear that it begins with a "yuh" sound that leads into the vowel ("oo"). That's the sound of the letter y in its consonant form (as opposed to, say, the "ee" sound of y at the end of the word party). So even though the word usual begins with a letter that usually represents a vowel sound, the sound we make when we say it is a consonant sound.

Here are some examples of how the use of a or an is dictated by the character of the initial sound of the word that follows the article:

Which of the following is not a usual ingredient of mayonnaise?
That's an unusual choice of gift.
That's a rather unusual gift.

Notice that a is used when the word rather, which starts with a consonant, separates the article from unusual, but an is used when the article directly precedes unusual.

The classic reason given for the difference in the use of a and an is that it's not easy to say two vowel sounds with a gap between them (e.g., "a unusual gift"). To distinguish between a and the word that follows it, you would need to use a glottal stop, or a near-stop — a break in the voicing of the kind that you hear if you say "Uh oh!" Far easier to insert an n, a consonant that is voiced at the front of the mouth, as a connecting sound between the two vowels.

That's the theory, anyway. In practice, I'm not sure the n insertion is as instinctive as all that. Witness the way children have to be taught not to say "a apple." And look at "Uh oh!" too: we seem to have no problem with using a glottal stop in this expression.

Just to complicate the picture further: some English speakers insert a connecting r between words (the idear of it) or syllables (That's a nice drawring) when the first word or syllable ends with a vowel sound and the second begins with one. Apart from occasions when dialect is represented in print, we don't write this interposed r the way we write an versus a.

But in the case of a and an, we do make a distinction in both the spoken and the written language — one that is governed by sound, not by spelling.

Wendalyn Nichols is the editor of the Copyediting newsletter and a commissioning editor of dictionaries for Cambridge University Press. She began as a freelance researcher, writer, and editor, then became a lexicographer and editor with the Longman Group. For four years she was the editorial director of Random House Reference and Information Publishing. She lives in New York, New York with her husband and young daughter. Follow her on Twitter @WendalynNichols and @Copyediting.