The post horn is a relatively simple kind of horn once played by postilions as a signal of the departure, arrival or approach of a coach. Mozart made brief use of the instrument in his Post Horn Serenade, and its sound was imitated by various composers, including Johann Sebastian Bach in his harpsichord Capriccio on the Departure of His Beloved Brother, which includes a Postilion Aria and a fugue on the sound of the post horn.