Bebop emerged in the early 1940s as a reaction against the commercial constraints of the Swing era. Musicians like Charlie Parker and Dizzy Gillespie wanted to shift jazz from dance music to 'musician's music.' Bebop is characterized by extremely fast tempos, complex chord progressions, and intricate improvisations based on harmonic structure rather than the melody. Unlike Swing, which used large orchestras, Bebop utilized small combos (quartets or quintets) and moved performances into intimate, late-night clubs. This style introduced 'flatted fifths' and other 'dissonant' notes that were revolutionary at the time. It challenged both the performers' technical skills and the listeners' ears, laying the foundation for all modern jazz that followed.