What Is a Glissando? Sliding Between Notes Instead of Stepping

A true continuous slide passes through every pitch between the start and the end, which requires an instrument whose pitch is not fixed to discrete positions. A trombone achieves it with the slide, a string instrument by moving a finger along the string, a voice by adjusting the vocal folds, and a few others including the theremin and the slide whistle by their construction. Instruments with fixed pitches cannot do it, so a pianist sweeping a hand across the keys produces a rapid succession of distinct notes rather than a continuous change, and a harpist does the same across the strings. Strictly the continuous version is a portamento and the stepped version is a glissando, though usage has blurred the terms and the word glissando is now applied to both, with the context usually making clear which is physically possible.

The technique differs completely between instruments:

• •Trombone, where the slide gives a genuinely continuous change over a limited range, and the instrument is the natural home of the effect
• •Strings, where a finger slides along the string, giving continuous change over any distance the hand can reach
• •Piano and harp, where the hand or finger sweeps across fixed pitches, producing the stepped version
• •Voice, which slides naturally and does so constantly in speech without anyone noticing
• •Clarinet and other woodwinds, where a combination of finger and embouchure control produces a partial slide, famously at the opening of Rhapsody in Blue
• •Timpani, where the pedal changes the tension while a note sounds, which is a genuine continuous change on a percussion instrument

The effect carries strong associations and composers use them deliberately. It suggests informality and speech, since the voice slides in ordinary talking and a sung slide sounds conversational or emotional rather than formal. It suggests instability, since the pitch is not settled anywhere. It can be comic, which is why the slide whistle became a stock sound effect and why an exaggerated trombone slide reads as a joke. It can be sensual or mournful depending on speed and direction. A slow slide upward builds expectation and a fast one downward deflates. In music outside the Western classical tradition the sliding between pitches is frequently not an effect at all but the normal way notes are connected, which means transcribing such music into notation designed for discrete pitches loses something central.

A handful of instances are so well known that they define the effect for most listeners. The clarinet opening of Rhapsody in Blue was written as a straight run and was turned into a slide by the clarinettist at rehearsal, which the composer kept, and it is now imitated by every player who performs it. Orchestral string slides are a staple of film scoring for tension and for horror. Trombone slides supply comedy and, in early jazz, an expressive vocabulary drawn directly from speech. Harp sweeps are so strongly associated with dreams and transitions that they function as a cue rather than as music. Electric guitar uses a slide bar to produce continuous pitch change, which came into blues and country from earlier Hawaiian practice. The wide familiarity of these examples is itself a reason composers use the effect sparingly, since it arrives carrying associations that are hard to suppress.

Notation for the effect is a compromise, since staff notation is built for discrete pitches and a continuous change has no natural representation. The convention is to write the starting and ending notes and join them with a straight or wavy line, with a word indicating which kind is intended. That leaves the shape of the slide unspecified, including whether it is even, whether it accelerates and exactly when it begins, all of which change the result substantially and all of which are left to the performer. Some composers have added instructions or drawn shaped lines to specify more. The limitation is a specific instance of a general one, since notation records a selection of what a performance contains and a great deal that matters, including timing nuance, dynamic shaping and timbre, is transmitted by tradition and by ear rather than by the page.

A genuinely continuous slide needs an instrument without fixed pitches, so a trombone and a string can do it and a piano cannot, producing rapid discrete notes instead. Strictly the continuous version is a portamento, though usage has merged the terms. The effect suggests speech, instability or comedy depending on speed and direction, and notation leaves its shape entirely to the performer.