In the theatre, perhaps the first lasting line-up was that for Britten's English Opera Group, formed in 1947; in the concert hall, it was Friedrich Cerha's ensemble Die Reihe, founded in 1958 - to be followed by the London Sinfonietta (1968), the New York-based Speculum Musicae (1971), the Schonberg Ensemble of Amsterdam (1974), the Ensemble InterContemporain in Paris (1977), and many more.The effect of this post-war promotion of the variable ensemble of virtuoso soloists as a central medium of musical thought has been vastly to expand the range of creative response: from the tranced stasis of Feldman to the hypercomplexity of Ferneyhough in the concert hall; from the hieratic orientalism of Britten to the surreal absurdity of Kagel on the stage; and, since the appearance of Varese's Deserts in 1954, the rapid evolution in combining taped sounds, electronic modulation and "real time" computer operations with live ensembles. When Schoenberg scored his "thrice times seven settings" Pierrot Lunaire (1912) for voice, flute (doubling piccolo), clarinet (doubling bass-clarinet), piano, violin (doubling viola) and cello, he was well aware similar ensembles could be heard in tea-rooms and spas all over Europe. Yet the unprecedented range of colour and expression he drew from unconventional vocal techniques, extremes of instrumental register and the ceaseless permutation of his forces proved so lastingly influential that it is hard not to hear Walton's Facade - that naughty adumbration of classical rap - as a Pierrot Lunaire for the 1920s; or Boulez's glittering serial Le marteau sans maitre, Kurtag's fevered Messages of the Late Miss RV Troussova and Goehr's stoical Sing, Ariel as the Pierrot Lunaires respectively of the 1950s, 1980s and 1990s.Indeed, over the decades, the ensemble concept has begun to throw up not only its own genres, such as the chamber concerto, the instrumental song-cycle and various forms of music-theatre, but a whole sequence of composers, from Webern, Varese, Wolpe, Dallapiccola and Lutyens down to such current figures as John Woolrich and Simon Holt, whose central development is to be traced not through their large-scale orchestral or operatic works on the one hand or standard chamber pieces on the other, but through their richly varied ensemble works.Given that richness, it is surprising that, up at least until the 1950s, promoters of this repertory were still mostly obliged to hire performers piecemeal by the date, and that specialist ensembles were not established earlier. Isolated in the United States, Charles Ives confined such polymetrical experiments as his scherzo Over the Pavements (1906) to ad hoc ensembles of players he could afford to hire, in order to hear them at all; while Igor Stravinsky, stuck in Switzerland during the First World War and compelled to make the best of local talent, came up with that vastly influential music-theatre score for seven players, The Soldier's Tale (1918).In the 1920s, the emergence of a popular ensemble culture in the form of jazz became a further stimulus to the Paris of Les Six and the Berlin of Hindemith and Weill. And, by this time, certain ensemble classics were beginning to inspire traditions of their own.

For some, the immediate concern was to get away from the post-Wagnerian cult of the kolossal, and Schoenberg's First Chamber Symphony (1906) already compresses the extremes of late Romanticism into a 20-minute structure for just, as he put it, "15 solo instruments" Other composers simply adapted to circumstances. Hence the increasing emergence of works for unusual, even unique, selections of instruments.Not that composers of what we now regard as the first ensemble classics necessarily thought in such grand historical terms. With the rise of a more commercially-minded bourgeoisie, composers found themselves released into the market and obliged to compete with one another.The result was a progressive abandonment of the norms of procedure and taste represented by the old genres in favour of self-expression and the pursuit of novelty. By the beginning of the 20th century, the notion that everything in a new piece, not least its scoring, should be derived from an original idea, was on the way to becoming axiomatic. Accordingly, composition was dominated by the concept of genres - the set forms of the liturgy, of court ceremonial, of private instruction or public celebration. True, what they played - being mostly learnt by ear or improvised - is lost.

But music survives for all kinds of Renaissance consorts, Baroque concertante groupings and Classical bands for dances and serenades. Even among the more standardised concert and chamber line-ups of the 19th century, we can find a scattering of individualised ensemble works: the octets of Schubert and Mendelssohn, for instance, are very different in scoring and character, and Saint-Saens was happy to add a saucy trumpet to the piano and strings of his Septet of 1881.So why do we tend to think of such flexible ensembles of instrumental - and, where necessary, vocal - virtuosi as the London Sinfonietta as essentially 20th-century? Part of the answer is doubtless socio-economic. Up to the end of the 18th century, most composers were in the service of church, court, great house or municipality. Question: when is an orchestra not an orchestra? Answer: when it's a variable ensemble. As the London Sinfonietta nears its 30th birthday, Bayan Northcott assesses the past and future possibilities of a peculiarly 20th-century medium. Of course there were always ensembles in Western music, as the cathedral carvings and illuminated manuscripts of the Middle Ages remind us.