pressure of the reed organ can be directly controlled by varying the speed the bellows are operated with the feet, providing a means of producing softer or harsher tones. Harmoniums (where the bellows provided a positive pressure) usually had an air reservoir to reduce the effort needed to pump the bellows when many stops were engaged. However, some instruments had a system to bypass the reservoir if the player wanted more direct control. Reed organs often had a form of swell shutter mechanism. The bellows system and reed ranks were contained in a wooden frame. By covering this frame, a swell box was created. A single shutter in the top of the box, inside the organ case, allowed the volume to be controlled. Since the player's feet were needed to operate the bellows, the swell shutter was controlled by a lever operated by the player's knee. The lever operated horizontally, and the player pushed their knee towards the side of the instrument to open the shutter. The lever returned to the 'closed' position on a spring or a locking mechanism could be engaged to hold the shutter open. Subsequently, the electrically powered reed organ, and later the electronic organ, allowed the bellows pedals to be replaced with an expression pedal, allowing the organist to effect a more substantial change in volume more easily than was possible with the manually pumped instrument. Electronic organs The style of popular organ music of the 20th and 21st centuries such as jazz is highly dynamic and requires constant use of the expression pedal in a fashion very different from that of classical organ literature. This tendency increased with the arrival of spinet organs and modern synthesizers, which offset the expression pedal and reduced the size of the pedalboard. These changes allowed the organist to keep the right foot constantly on the expression pedal, while playing the pedalboard with only the left foot. This ability encouraged organists to operate the expression pedal more often during playing. To take advantage of this style of playing, some expression pedals on modern electronic organs are equipped with toe switches, which allow the organist to make quick registration changes without removing the foot from the pedal. Expression pedals may be non-linear in response, meaning that slight pressure changes may cause a greater proportional change in volume than a more complete depression. In this regard, each organ tends to be