- Errors involving the indefinite articles a and an are almost certainly more often a consequence of haste and carelessness than of ignorance. They are especially common when numbers are involved, as here: "Cox will contribute 10 percent of the equity needed to build a $80 million cable system" or "He was assisted initially by two officers from the sheriff ’s department and a FBI agent." When the first letter of an abbreviation is pronounced as a vowel, as in "FBI," the preceding article should be an, not a.
Bryson’s dictionary for writers and editors. 2013.