This unusual design incorportes four piston valves as an F/Bb "double" horn. The horn stands in F while depressing the thumb valve places it in Bb as on a standard double horn. There is, however, only one set of valve tuning slides and they are of proper length for the F horn. The thumb piston merely bypasses a section of tubing (including an auxiliary F tuning slice) to raise the horn to Bb. Thus the standard Bb fingerings are substantially flat due to the length of the valve slides. Some adjustment can be accomplished by using the F tuning slide to either bring the open Bb or the valved Bb notes (but not both) more nearly in tune with the F side. This horn is similar in design to the compensating double Boosey "Sotone" in another collection.