The French horn is known for its wide, flared bell and the fact that the player keeps their right hand inside it. This practice began before the invention of valves (the 'natural horn' era). By moving the hand into the bell, the player can change the length of the air column and slightly alter the pitch, allowing them to play notes that weren't in the natural harmonic series of the instrument. This is known as 'hand-stopping.' Even with the addition of modern valves, the hand remains in the bell to modify the tone, making it more mellow and dark. If the player pushes their hand deep into the bell to block the air almost entirely, it produces a 'stopped' sound, which is thin, nasal, and metallic. Because the bell faces backward, the sound reflects off the walls of the concert hall, which contributes to the horn's unique, blending orchestral color.