Bebop emerged in the early 1940s as a reaction against the commercial constraints of the Swing era and Big Band music. Musicians like Charlie Parker, Dizzy Gillespie, and Thelonious Monk wanted to shift jazz from danceable entertainment to a high-art form of listening. Bebop is characterized by extremely fast tempos, complex chord progressions, and intricate improvisations based on harmonic structure rather than the melody. Unlike Swing, which featured large ensembles, Bebop usually utilized small combos (quartets or quintets) consisting of saxophone, trumpet, piano, bass, and drums. The style introduced 'flatted fifths' and 'extensions' that were considered dissonant at the time. This genre laid the foundation for modern jazz, challenging performers to reach the peak of technical virtuosity. It also moved jazz performances from large ballrooms to intimate, late-night clubs in Harlem and 52nd Street in New York City.