Double horn
F horn and B♭ horn in one instrument.
What it means
The double horn unites two instruments in one: a thumb valve switches between a set of tubing in F and a shorter one in B♭. Both share the mouthpiece, the bell, and the three main valves but have valve slides of their own and thus a tuning of their own. It is notated uniformly for horn in F, sounding a perfect fifth lower than written; which side the player uses is a purely technical decision and changes nothing in the notation.
Musical use
The reason for this construction lies in the partials. On the long F side the notes in the high register lie very close together, which makes the sound soft but the hitting harder. The shorter B♭ side has partials lying farther apart and is markedly more secure in the high register but sounds somewhat slimmer and more direct. Since the early twentieth century, the double horn has therefore been the standard instrument in the orchestra: it combines the tone colour of the F horn in the low register with the accuracy of the B♭ horn in the high register. For the repertoire this means that even demanding high parts can be executed reliably. In addition there are triple horns with an added high side.
Performance practice
For tricky passages, decide deliberately on which side you play, and write it in the part; unplanned switching in the middle of a phrase produces audible jumps of colour. The transition range lies, depending on player and instrument, at about the notated g′, and it is precisely there that experimenting is worthwhile. Tune both sides separately and check the valve slides regularly, because a single out-of-tune side is hard to hear. Practise slurs across the switching point until the change remains tonally inconspicuous. Keep the thumb lever moving smoothly and empty the condensation from both sets of tubing.
Bear in mind that the two sides demand different amounts of air: the F side responds more sluggishly and needs more calm, the B♭ side reacts faster. Whoever switches constantly without considering this produces uneven volumes. Practise the same passages on both sides and compare sound and security. For quiet entries in the high register the B♭ side is usually more reliable, for soft melodies in the middle register the F side. Keep the thumb lever well maintained, because a sticking change in the middle of a phrase spoils everything.