The trio recordings of 1946 show a novel approach in the linear interaction between piano and guitar, resulting in counterpoint, polyrhythm, and superimposed harmonies. The sextet recordings of 1949 are notable for coherent ensemble playing and soloing, and the free group improvisations based on spontaneous group interactions and the contrapuntal principle. In the 1950s Tristano employed an advanced concept in jazz improvisation called side-slipping, or outside playing, which creates a form of temporary bitonality when chromatic harmony is superimposed over the standard harmonic progressions. Tristano intensified his use of counterpoint, polyrhythm, and chromaticism in the 1960s[.]