A small brass instrument, originally with a fundamental about b ♭′, used in the past by postillions and guards on mail coaches. In Germany it reached its standard form, circular and with three turns, in the late 18th century. As it graduated to use in concert music it acquired crooks, keys and finally valves. Telemann and Bach imitated it. Mozart, Beethoven and Mahler actually wrote for it. The English instrument, long and straight, in A or A♭, is still made; it is used in performances of Koenig's famous Post Horn Galop (1844).
