<P> A common view of what a jazz soloist does could be expressed thus: as the harmonies go by, he selects notes from each chord, out of which he fashions a melody . He is free to embellish by means of passing and neighbor tones, and he may add extensions to the chords, but at all times a good improviser must follow the changes...(However), a jazz musician really has several options: he may reflect the chord progression exactly, he may "skim over" the progression and simply elaborate the background harmony, or he may fashion his own voice - leading which may clash at some points with the chords the rhythm section is playing . </P> <P> With the notable exception of liturgical improvisation on the organ, the first half of the twentieth century is marked by an almost total absence of actual improvisation in art music . Since the 1950s, some contemporary composers have placed fewer restrictions on the improvising performer, using techniques such as vague notation (for example, indicating only that a certain number of notes must sound within a defined period of time). New Music ensembles formed around improvisation were founded, such as the Scratch Orchestra in England; Musica Elettronica Viva in Italy; Lukas Foss Improvisation Chamber Ensemble at the University of California, Los Angeles; Larry Austin's New Music Ensemble at the University of California, Davis; the ONCE Group at Ann Arbor; the Sonic Arts Group; and Sonics, the latter three funding themselves through concerts, tours, and grants . Significant pieces include Foss Time Cycles (1960) and Echoi (1963). </P> <P> Other composers working with improvisation include Richard Barrett, Benjamin Boretz, Pierre Boulez, Joseph Brent, Sylvano Bussotti, Cornelius Cardew, Jani Christou, Douglas J. Cuomo, Alvin Curran, Stuart Dempster, Hugh Davies, Karlheinz Essl, Mohammed Fairouz, Rolf Gehlhaar, Vinko Globokar, Richard Grayson, Hans - Joachim Hespos, Barton McLean, Priscilla McLean, Stephen Nachmanovitch, Pauline Oliveros, Henri Pousseur, Todd Reynolds, Terry Riley, Frederic Rzewski, William O. Smith, Manfred Stahnke, Karlheinz Stockhausen, Toru Takemitsu, Richard Teitelbaum, Vangelis, Michael Vetter, Christian Wolff, Iannis Xenakis, Yitzhak Yedid, La Monte Young, Frank Zappa, Hans Zender, and John Zorn . </P> <P> In the 1960s The Grateful Dead gained popularity and bestowed a name on the "jam" genre . Since the 1980s, bands like Phish, Widespread Panic, moe., Umphrey's McGee, Max Creek, and The String Cheese Incident have used musical improvisation extensively; indeed, for the more devoted followers of any band, these extended improvisational segments--jams--are a large part of what makes a live show so special . The jam band scene has also seen the rise of "jamgrass" with bands like Hot Buttered Rum, Cornmeal and Yonder Mountain String Band, along with the rise of Livetronica with bands like The Disco Biscuits, Lotus, The New Deal, and STS9 (Sound Tribe Sector 9), most of whom feature improvisation in their music . </P>

Improvisation as the central tenet of the jazz aesthetic