The origins of the french horn are found in the natural horn. The need for pratical convenience led to the long pipe being formed into the familiar circular shape. Without modern day valves and slides, the natural horn is restricted to the overtone series determined by the length of the tubing. It is possible to create additional pitches between the natural harmonics by closing off the bell with the right hand.
natural horn with your choice of 4 crooks:
B, A, G, F, E, Es, D, C