The varying of the volume or intensity of music. This is done by operating the controls of an instrument (e.g., the pedals of a piano), by opening and closing swell or crescendo shutters, by varying the suction level in a piano (low suction = soft music; high suction = louder music), by having pins of different length in a cylinder-operated instrument and by other means. An instrument is said to have "expression" if the musical performance can be varied in one of these ways. A music roll or music book may have expression cut in it to operate such controls automatically, or they may be operated by hand (as with the expression shutter of an organette).