An expression pedal is an important control found on many organs and synthesizers that allows the volume of the sound to be manipulated. Because the source of power with a pipe organ and electronic organs is not generated by the organist, the volume of these instruments has no relationship with how hard its keys or pedals are struck; i.e., the organ produces the same volume whether the key or pedal is depressed gently or firmly. Moreover, the tone will remain constant in pitch, volume, and timbre until the key or pedal is lifted, at which point the sound stops. The expression pedal grants the organist control over the external source of power, and thus the volume, of the instrument, while leaving his or her hands free. This system of dynamic control is completely distinct from the act of adding stops (in the case of pipe organs) or pulling more drawbars (in the case of digital organs and synthesizers). Furthermore, the expression pedal can influence the volume (and, to a lesser degree, the timbre) of a note while it is being played; unlike other instruments, in which the note typically decays after it is first sounded, the organist can increase the strength of a chord or note as it sounds merely by increasing her pressure on the expression pedal.
The modern expression pedal itself is a large pedal, roughly resembling an oversized automobile accelerator, either partially or fully recessed within the organ console and located either directly above or to the right of the organ's pedalboard. As the pedal is pressed forward with the toes, the volume of the sound is increased; as it is depressed with the heel, the volume is decreased.
An important consideration to remember is that no matter how well a swell box is designed, the sound of the pipes is altered by their enclosure. Even when the shutters are fully opened, the pipes do not speak as clearly into the room as they would if they were otherwise unenclosed.
On pipe organs, the expression pedal should not be confused with the crescendo pedal, which progressively adds stops as it is opened.
Correspondence to The Musical Times in 1916 debates the merits of both the ratchet lever and balanced pedal systems of expression. One writer suggests that balanced expression pedals are either too sensitive or not sensitive enough and are unable to produce effective sforzandos (though many improvements have been made since this letter was written in 1916), and that he knows many organists who are having balanced expression pedals removed. One organist most open to the change suggests that real crescendos and diminuendos are not possible with a ratchet swell lever, as the notches provided are always either just under or just above the required dynamic level. Furthermore, he states that the balanced expression pedal affords the ease of use of either foot, whereas the previous correspondent desired two ratchet levers, one at either side of the pedalboard.
Reed organs and harmoniums of the late nineteenth and early twentieth centuries featured a pair of bellows pedals at the base of the instrument. When the pedals were pumped up and down, air was drawn across the organ’s reeds, producing sound. This capability made the harmonium widely available to homes and small churches, though the dynamic range tended to be limited (not to mention that the organist would eventually tire from pumping). However, these free-reed organs had several ways of controlling their volume and expression. Unlike a pipe organ with a blower, the wind 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 enagaged 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.
The style of popular organ music of the twentieth and twenty-first 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 rather distinct.