Hand Position and Mutes
The use of the hand inserted into the bell of the horn is unique to the instrument. The earliest types of horns played only the notes of the harmonic series, but it was discovered that the insertion of the hand could bend the pitch to enable intermediate notes. On the modern valve horn, hand technique has become a timbral modifier as well as a means to change pitch.
Composers like Mozart and Beethoven wrote superbly well for the horn and and understood the nuances of hand horn technique. They would write to exploit the technique to maximum effect, rather than seeing it as a hindrance. They would actually write the music around this technique, so that for certain phrases that included stopped notes the change in sound would mean something different musically.