Also known as bebop, Bop was a radical new music that developed gradually in the early '40s and exploded in 1945. The main difference between bop and swing is that the soloists engaged in chordal (rather than melodic) improvisation, often discarding the melody altogether after the first chorus and using the chords as the basis for the solo. Ensembles tended to be unisons, most jazz groups were under seven pieces, and the soloist was free to get as adventurous as possible as long as the overall improvisation fit into the chord structure. Since the virtuoso musicians were getting away from using the melodies as the basis for their solos (leading some listeners to ask "Where's the melody?") and some of the tempos were very fast, bop divorced itself from popular music and a dancing audience, uplifting jazz to an art music but cutting deeply into its potential commercial success. Ironically the once-radical bebop style has become the foundation for all of the innovations that followed and now can be almost thought of as establishment music. Among its key innovators were altoist Charlie Parker, trumpeter Dizzy Gillespie, pianist Bud Powell, drummer Max Roach, and pianist/composer Thelonious Monk.