of the string is silenced by strips of damping cloth. By applying a rocking pressure up and down the key with the finger, a performer can slightly alter the vibrating length of the string itself, producing a vibrato quality known as bebung. While the vibrato on fretless string instruments such as the violin typically oscillates in pitch both above and below the root note, clavichord bebung only produces pitches above the note. Sheet music does not often explicitly indicate bebung. Composers generally let players apply bebung at their discretion. When sheet music does indicate bebung, it appears as a series of dots above or below a note; the number of dots indicates the number of finger movements. On electronic keyboards and synthesizers, pressure sensitivity is usually called aftertouch. The vast majority of such instruments use only channel aftertouch: that is, one level of pressure is reported across the entire keyboard, which affects either all notes pressed (even ones not being pushed into aftertouch) or a subset of the active notes in some instruments that allow this level of control. A minority of instruments have polyphonic aftertouch, in which each individual note has its own sensor for pressure that enables differing usage of aftertouch for different notes. Aftertouch sensors detect whether the musician is continuing to exert pressure after the initial strike of the key. The aftertouch feature allows keyboard players to change the tone or sound of a note after it is struck, the way that singers, wind players, or bowed instrument players can do. On some keyboards, sounds or synth voices have a preset pressure sensitivity effect, such as a swell in volume (mimicking a popular idiomatic style of vocal performance with melodies) or the addition of vibrato. On some keyboards - a good example of such an instrument being Yamaha's programmable synthesiser-workstation, the Yamaha EX5 - the player can select the effects to which aftertouch applies. This allows a performer to custom-tailor the effect that they desire. It may also facilitate the imitation of various non-keyboard instruments. For example, a keyboardist who wishes to imitate the sound of a heavy metal guitar solo could use a distortion guitar sound, and then set the aftertouch feature to apply a pitch bend to the note. Displacement sensitivity A third form of sensitivity is displacement sensitivity. Displacement-sensitive keyboards are often found on organs. Most mechanical organs, and some electrically actuated organs,