Pitch is controlled through the combination of the following factors: speed of air through the instrument (controlled by the player's lungs and thoracic diaphragm); diameter and tension of lip aperture (by the player's lip muscles—the embouchure) in the mouthpiece; plus, in a modern French horn, the operation of valves by the left hand, which route the air into extra sections of tubing. Most horns have lever-operated rotary valves, but some, especially older horns, use piston valves (similar to a trumpet's) and the Vienna horn uses double-piston valves, or pumpenvalves. The backward-facing orientation of the bell relates to the perceived desirability to create a subdued sound in concert situations, in contrast to the more piercing quality of the trumpet. A horn without valves is known as a natural horn, changing pitch along the natural harmonics of the instrument (similar to a bugle). Pitch may also be controlled by the position of the hand in the bell, in effect reducing the bell's diameter. The pitch of any note can easily be raised or lowered by adjusting the hand position in the bell. The key of a natural horn can be changed by adding different crooks of different lengths.