The natural horn is the purest of all brass instruments.  Built without valves, it consists of one long loop plus extra crooks manually inserted by the player to change keys.  Notes outside of the key are played by "stopping" the horn with adjustment of the right hand position.  The Mozart Rondo played on natural horn is a good example of this technique.  When you already own a double and triple horn and a Wagner tuba, you know you must also have a natural horn.