Hand-stopping is a technique by which a natural horn can be made to produce notes outside of its normal harmonic series. By inserting the hand, cupped, into the bell, the player can reduce the pitch of a note by a semitone or more. This, combined with the use of crooks changing the key of the instrument, allowed composers to write fully chromatic music for the horn before the invention of piston and valve horns in the early 19th Century.