Use an before any word beginning with a vowel sound: an officer, an honorary degree. Because the h in the word historical and its variations is pronounced, those words take an a: an hourlong talk at a historical society. Likewise, an initialism (whose letters are sounded out individually) may be paired with one article, while an acronym (which is pronounced as a word) beginning with the same letter is paired with the other: an HTML document describing a HUD program.