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Glissando (plural: glissandi) is a musical term that refers to either a continuous sliding from one pitch to another (a "true" glissando), or an incidental scale played while moving from one melodic note to another (an "effective" glissando).
Musical instruments with continuously variable pitch can effect a true glissando over a substantial range. These include unfretted stringed instruments (such as the violin, viola, cello and double bass and fretless bass guitars), stringed instruments with a way of stretching the strings (such as the guitar or sitar), wind instruments without valves or stops (such as the trombone or slide whistle), timpani (kettledrums), electronic instruments (such as the theremin, synthesizers and keytars), the water organ, and of course the human voice.
True glissandi can be produced over a limited range on most instruments; for example, fretted stringed instruments (such as the guitar or mandolin) can effect a glissando by pushing the string across the fingerboard or by using a slide. This is commonly called note 'bending' as opposed to a glissando. Brass and wind instruments such as the flute or trumpet can effect a similarly limited glissando by altering the breath pressure, while the clarinet can achieve this by slowly dragging fingers off tone holes. Tunable percussion instruments such as the drum or conga can effect small glissandi by applying or releasing pressure on the head while striking.
On some instruments (e.g., piano, harp), a bending of the tone or continuous sliding is not possible. As a substitute, the player can play a number of adjacent notes in rapid succession, so that the audible result somewhat resembles a true glissando. For example, on a piano, the player can slide his/her thumb or fingers across the white or black keys, producing either a C major scale or an F# major pentatonic (or their relative modes). On a harp, the player can slide his/her finger across the strings, quickly playing the separate notes. Wind, brass and fretted stringed instrument players can effect an extremely rapid chromatic scale (ex: sliding up or down a string quickly on a fretted instrument), giving the same effect. These latter techniques are commonly referred to and notated as glissandi in scores and sheet music, although technically they are only "effective" glissandi.
See also: musical terminology