technique of a circulating bass line and harmonic sequence, invented and much used in 17th- and 18th-century baroque music but also central to the idea of jazz changes, predominates in most of Piazzolla's mature compositions. Another clear reference to the baroque is the often complex and virtuosic counterpoint that sometimes follows strict fugal behavior but more often simply allows each performer in the group to assert his voice. A further technique that emphasises this sense of democracy and freedom among the musicians is improvisation, that is borrowed from jazz in concept, but in practice involves a different vocabulary of scales and rhythms that stay within the parameters of the established tango sound-world. Pablo Ziegler has been particularly responsible for developing this aspect of the style both within Piazzolla's groups and since the composer's death. With the composition of Adiós Nonino in 1959, Piazzolla established a standard structural pattern for his compositions, involving a formal pattern of fast-slow-fast-slow-coda, with the fast sections emphasizing gritty tango rhythms and harsh, angular melodic figures and the slower sections usually making use of the string instrument in the group and/or Piazzolla's own bandoneon as lyrical soloists. The piano tends to be used throughout as a percussive rhythmic backbone, while the electric guitar either joins in this role or spins filigree improvisations; the double bass parts are usually of little interest, but provide an indispensable rugged thickness to the sound of the ensemble. The quintet of bandoneon, violin, piano, electric guitar and double bass was Piazzolla's preferred setup on two extended occasions during his career, and most critics consider it to be the most successful instrumentation for his works. This is due partly to its great efficiency in terms of sound – it covers or imitates most sections of a symphony orchestra, including the percussion, which is improvised by all players on the bodies of their instruments – and the strong expressive identity it permits each individual musician. With a style that is both rugged and intricate, such a setup augments the compositions' inherent characteristics. Despite the prevalence of the quintet formation and the ABABC compositional structure, Piazzolla consistently experimented with other musical forms and instrumental combinations. In 1965 an album was released containing collaborations between Piazzolla and Jorge Luis Borges where Borges's poetry was narrated over very avant-garde music by Piazzolla including the use of dodecaphonic (twelve-tone) rows, free non-melodic improvisation on all instruments, and