Horn (musical instrument)
The horn has a conical bore.  This means that the tubing gradually becomes wider (a trumpet and trombone have a cylindrical bore): the tubing is the same thickness all the way along.  The modern horn has 6.4 metres of tubing.   It makes a beautiful, warm sound, but it is quite hard to play because the mouthpiece is small and the harmonics (the notes that can be played by changing the lip pressure) are very close together.  The horn is a transposing instrument, usually in F (a written C sounds like the F below).