The bass trumpet is the rarest of all trumpets.  Created in 18th century Germany, it is used in brass bands, and as an auxiliary instrument in the orchestra.  Wagner, Stravinsky and others wrote for the bass trumpet.  Bass trumpets may be pitched in Bb, C or Eb, and some may convert between two keys.  The traditional German style bass trumpet may have 3 or 4 rotary valves, and the American style uses piston valves.  Playing the bass trumpet is usually the responsibility of a wayward trombone player, and the tone desired is reminiscent of the French horn.