Holton Quality The H200 Descant Horn This instrument is a "full" (not compensating) double, requiring no compromise of intonation on valve combination, as is the case with compensating designs. The fifth valve is a Bb-F rotor. The fourth valve is a muting valve that lowers the horn 3/4 step to bring stopped horn mutes into tune; no transposing is necessary. Extra fourth slide puts Bb horn in the key of A, so written E horn parts do not have to be transposed.