 "Progressive jazz" was a broad category of music that included bebop-influenced "art music" arrangements used by big bands such as those led by Boyd Raeburn, Charlie Ventura, Claude Thornhill, and Stan Kenton, and the cerebral harmonic explorations of smaller groups such as those led by pianists Lennie Tristano and Dave Brubeck.Voicing experiments based on bebop harmonic devices were used by Miles Davis and Gil Evans for the groundbreaking "Birth of the Cool" sessions in 1949 and 1950.Musicians who followed the stylistic doors opened by Davis, Evans, Tristano, and Brubeck would form the core of the cool jazz and "west coast jazz" movements of the early 1950s.