The post horn (also posthorn, post-horn, or coach horn) is a valveless cylindrical brass or copper instrument with cupped mouthpiece, used to signal the arrival or departure of a post rider or mail coach. It was used especially by postilions of the 18th and 19th centuries.
The instrument commonly had a circular or coiled shape with three turns of the tubing, though sometimes it was straight. It is therefore an example of a natural horn. The cornet was developed from the post horn by adding valves.
Mozart, Mahler, and others incorporated the instrument into their orchestras for certain pieces. On such occasions, the orchestra's horn player usually plays the instrument. One example of post horn use in modern classical music is the famous off-stage solo in Mahler's Third Symphony. Due to the scarcity of this instrument, however, music written for it is usually played on a trumpet or flugelhorn.
The instrument is still used as the logo of national post services in many countries.