An expression pedal is an important control found on many organs

Organ (music)

The organ , is a keyboard instrument of one or more divisions, each played with its own keyboard operated either with the hands or with the feet. The organ is a relatively old musical instrument in the Western musical tradition, dating from the time of Ctesibius of Alexandria who is credited with...

 and synthesizers, as well as in the pedal steel guitar

Pedal steel guitar

The pedal steel guitar is a type of electric guitar that uses a metal bar to "fret" or shorten the length of the strings, rather than fingers on strings as with a conventional guitar. Unlike other types of steel guitar, it also uses pedals and knee levers to affect the pitch, hence the name "pedal"...

, that allows the volume

Loudness

Loudness is the quality of a sound that is primarily a psychological correlate of physical strength . More formally, it is defined as "that attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud."Loudness, a subjective measure, is often...

 of the sound to be manipulated. Because the source of power with a pipe organ

Pipe organ

The pipe organ is a musical instrument that produces sound by driving pressurized air through pipes selected via a keyboard. Because each organ pipe produces a single pitch, the pipes are provided in sets called ranks, each of which has a common timbre and volume throughout the keyboard compass...

 and electronic organ

Electronic organ

An electronic organ is an electronic keyboard instrument which was derived from the harmonium, pipe organ and theatre organ. Originally, it was designed to imitate the sound of pipe organs, theatre organs, band sounds, or orchestral sounds....

s 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.