The Natural Horn
Before instrument manufactures developed the technology required for pistons or rotors, the horn used multiple 'crooks' to change the key of the horn. This is one of the reasons that horn players today must do so much transposition when playing very old arragements. The horn player used his/her hand in the bell of the horn to play notes that where between the natural harmonics of whatever crook was currently in use.
There are some folks that still use a natural horn at times to play certain arragements as authentic as possible.