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In music, a glissando [ɡlisˈsando] (plural: glissandi, abbreviated gliss.) is a glide from one pitch to another. It is an Italianized musical term derived from the French glisser, to glide. In some contexts it is distinguished from the continuous portamento. Some colloquial equivalents are slide, sweep (referring to the 'discrete glissando' effects on guitar & harp respectively), bend, smear, rip (for a loud, violent gliss to the beginning of a note), lip (in jazz terminology, when executed by changing one's embouchure on a wind instrument), or falling hail (a glissando on a harp using the back of the fingernails).
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From the standpoint of musical acoustics and scientific terminology, some instruments, such as slide trombones, unfretted bowed-string instruments, guitars played with slides and, of course, slide whistles, can change the frequency of their notes continuously, while others, notably acoustic keyboard instruments, are restricted to quantized (stepped) changes in pitch. (The clavichord's Bebung is the one exception, but that is essentially ornamentation of a single pitch, not a glide.) Some instruments, such as the clarinet and saxophone, can produce a continuous pitch (frequency) change, although their characteristic design is to provide distinct pitches.
Glissando vs. portamento
Prescriptive attempts to distinguish the glissando from the portamento by limiting the former to the filling in of discrete intermediate pitches on instruments like the piano, harp, and fretted stringed instruments have run up against established usage of instruments like the trombone and timpani. The latter could thus be thought of as capable of either 'glissando' or 'portamento', depending on whether the drum was rolled or not. The clarinet gesture that opens Rhapsody in Blue could likewise be thought of either way: it was originally planned as a glissando (Gershwin's score labels each individual note) but is in practice played as a portamento though described as a glissando.
'Discrete glissando'
On some instruments (e.g., piano, harp, xylophone), discrete tones are clearly audible when sliding. For example, on a keyboard, a player's fingertips can be made to slide across the white keys or over the black keys, producing either a C major scale or an F♯ major pentatonic (or their relative modes); or, by performing both at once, it is possible to produce a full chromatic scale, but this is difficult. On a harp, the player can slide their finger across the strings, quickly playing the scale (or on pedal harp even arpeggios such as C♭-D-E♯-F-G♯-A♭-B). Wind, brass, and fretted-stringed-instrument players can perform an extremely rapid chromatic scale (e.g., sliding up or down a string quickly on a fretted instrument).
Maurice Ravel's piece Alborada del Gracioso contains notable piano glissando passages in thirds executed by the right hand.
Arpeggio effects (likewise named glissando) are also obtained by bowed strings (playing harmonics) and brass, especially the horn.
'Continuous glissando' or portamento
Musical instruments with continuously variable pitch can effect a portamento over a substantial range. These include unfretted stringed instruments (such as the violin, viola, cello and double bass, and fretless guitars), stringed instruments with a way of stretching the strings (such as the guitar, veena, or sitar), a fretted guitar or lap steel guitar when accompanied with the use of a slide, wind instruments without valves or stops (such as the trombone or slide whistle), timpani (kettledrums), electronic instruments (such as the theremin, the ondes martenot, synthesizers and keytars), the water organ, and the human voice.
Brass and woodwind instruments such as the trumpet or flute can effect a similar limited slide by altering the lip pressure (trumpet) or a combination of embouchure and rolling the head joint (flute), while the clarinet and some models of flute can achieve this by slowly dragging fingers off tone holes or changing the oral cavity's resonance by manipulating tongue position, embouchure, and throat shaping.
Many electric guitars are fitted with a tremolo arm which can produce either a portamento, a vibrato, or a combination of both (but not a true tremolo despite the name). Tremolo is a repeated variation of loudness while holding pitch constant; vibrato is a repeated variation of pitch (frequency).