Classic Music: Taking a line for a walk: As Berio's Sequenzas receive a complete South Bank airing, Bayan Northcott looks at music where the soloist must go it alone

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As so often in 20th-century music, the initial breakthrough seems to have been made by Claude Debussy. Asked at a busy moment in 1913 for some incidental music to a play with an ancient Greek setting, he hastily responded with a two-minute solo for flute, eventually published under the title Syrinx. Intensely characteristic, its sinuously drooping phrases and velvety exploitation of the instrument's lowest notes have since seduced musical analysts almost as frequently as players and listeners. But the salient challenge of Syrinx was its projection as pure line - dispensing with, indeed questioning the whole notion of, accompaniment.

One needs to trawl the complete catalogues of the masters of the previous 150 years to realise just how radical this was. Of course, there were piano solos by the bushel, to say nothing of guitar, harp or organ pieces. But these are instruments which, out of their ranges of pitch, dynamics and colour, are able to sustain full textures - to accompany, as it were, their own ideas. Of unaccompanied pieces for genuinely monophonic instruments (including the human voice, which normally produces only one pitch at a time) one will find virtually nothing.

Admittedly, the earlier 18th century, when the transverse flute was celebrating its final triumph over the recorder, did yield a number of genuinely solo pieces, including a little Partita in A minor by J S Bach. But the Baroque ideal of melody ordained that the underlying harmony of a piece should be arpeggiated into the overall line, and a Bach vocal part or instrumental obbligato extracted from a full texture will, generally, make sense on its own. What Debussy was harking back to in Syrinx was rather a pre-contrapuntal, pre-harmonic culture, as imagined in the Ancient World, documented in the vast monophonic repertoire of the Middle Ages and still sustained in European folk music and some exotic traditions.

There were two reasons why this was bound to sound timely around 1913. Not only was the apparatus of composition in the vast structures of Mahler, Strauss, Schoenberg and Stravinsky growing complex beyond measure, but this very pace of change was threatening to destroy the old certainties of harmony, form and expression. If pieces in future were increasingly going to have to define their own languages as they went along, then at least a return to severely restricted means might enable this to be done coherently. Sure enough, a number of subsequent solo monodies - Stravinsky's raucously folkloristic Three Pieces for Clarinet (1919), for instance, or Britten's ingeniously pictorial Six Metamorphoses After Ovid (1951) for oboe - were to prove as central to the repertoires of their instruments as Syrinx has to the flute.

Though not only Syrinx - for in 1936 Edgard Varese brought forth his hugely influential solo Density 21.5. This, however, pursued an opposite course to the Debussy: mounting by angular degrees to the piercing heights of the range and punctuating its declamatory phrases with percussive taps on the instrument's keys. No doubt Luciano Berio was well aware of this pair of classics by the time he sat down to compose his Sequenza for the avant-garde flautist Severino Gazzelloni in 1958. Yet he managed to come up with something different again.

Granted, the six-minute result tends, these days, to sound a bit of a period piece in its continual, relentlessly irregular scattering of notes across all registers - so that neither a regular rhythm nor a sustained line ever quite manages to emerge. Yet, of all the products of that fragmentary Darmstadt aesthetic, it remains among the most elegant. Moreover, its widespread welcome among more adventurous flautists encouraged Berio to follow up over the years with a whole sequence of Sequenzas for other performers. Of the 11 to date, only two, admittedly, are purely monophonic: Sequenza III (1966), a volatile cross-cutting of sung phonemes, chatter and tongue-clicks conceived for the idiosyncratic virtuosity of Berio's first wife, the soprano Cathy Berberian; and Sequenza IX (1980) for clarinet, a slowly unfolding permutation of pitches that most nearly approaches traditional ideas of melody.

Sequenza VII (1969) for oboe, by contrast, is supplemented by an offstage drone on the note B, around which the instrument's jagged sallies dart and sting like gnats, while in Sequenza X (1984) for trumpet, a nearby piano is made to echo the more stentorian blasts by sympathetic resonance. Even the original flute Sequenza included a few so-called multiphonics - those curious chord effects the 1950s avant-garde was so excited to extract from single woodwind or brass instruments by special combinations of lip and fingering. And in the theatrical Sequenza V (1966) for trombone, inspired by the grotesque gestures of the legendary clown Grock, the player is required to generate different tones by simultaneously blowing and singing into the instrument.

What is as rigorously excluded from the entire series as from Debussy's Syrinx is any sense of accompaniment. Even the fiercely twangling Sequenza II (1963) for harp, and the densely chordal Sequenza IV (1966) for piano, plug away doggedly at single technical concerns. And the concept of the solo piece primarily as a study in aspects of virtuosity is particularly marked in Sequenza VI (1967) for viola, Sequenza VIII (1977) for violin and Sequenza XI (1988) for guitar, all of which feature dense barrages of thrumbed chords which only later give way to more linear phrases.

Comparing Berio's Sequenzas with other latterday sequences-in-progress of unaccompanied pieces, it is evident that his concentration on more recherche attributes of his chosen instruments goes far beyond the kind of traditional solo study as represented by the series of Fantasies which Sir Malcolm Arnold has been composing for a gamut of standard instruments - there is even a Fantasy for tuba - since 1966. On the other hand, Berio has never demanded the impossible, as some of Brian Ferneyhough's solo pieces deliberately do - the idea apparently being that, in struggling to realise the notational complexities of such pieces as Time and Motion Study I (1971) for bass clarinet, players will generate a special kind of expressive tension. Yet such transcendental studies differ again from the series of solo pieces Elliott Carter has recently been composing - such as Riconoscenza (1984) for violin, or Scrivo in Vento (1991) for flute. These tend to introduce, alternate and juggle many different types of material, proceeding like microcosms of his larger works.

All the same, it is Berio who has taken the most unexpected turn. Asked for a harp concerto back in 1964, he simply proceeded to elaborate an orchestral accompaniment around the more or less unchanged Sequenza II, calling the result Chemins I, and repeating the process 11 years later by adding a nimbus of string sonorities to his oboe Sequenza in Chemins IV. Meanwhile, the viola Sequenza had been successively lapped in chamber ensemble and full orchestral sonorities in 1967-8 as Chemins II and III.

Cynics might regard this as the musical equivalent of rediscovering the wheel: Berio invents concerto form. He himself emphatically distinguishes the process from any traditional interplay of soloist and orchestra, likening it rather to the layering of an aural onion. Those following the current South Bank Berio series will be able to judge whether the Sequenzas have indeed generated something new, for in addition to this Thursday's performance of the complete set (QEH, 7pm), next Saturday sees the British premiere (QEH, 7.45pm) of the latest Chemins, wrapped around the manic solo guitar of Sequenza XI.

Berio series: South Bank Centre, London SE1 (071-928 8800) to 14 May