The AP Stylebook’s guidelines for the use of a and an follow the traditional rule. Below is a summary of the AP Style a-an guidelines.
Use “a” before consonant sounds: a historic event, a one-year membership (in this case, the “one” begins with a “w” sound).
Use “an” before vowel sounds: an energy crisis, an honorary degree (the “h” is silent), an NFL record (sounds like it starts with the letter “e”).
For a more full discussion of the differences between the uses of “a” and “an” and some of the trouble words, please see our full page on the subject.