Bebop or bop is a style of jazz developed in the early to mid-1940s in the United States, which features compositions characterized by a fast tempo, complex chord progressions with rapid chord changes and numerous changes of key, instrumental virtuosity, and improvisation based on a combination of harmonic structure, the use of scales and occasional references to the melody.

Bebop developed as the younger generation of jazz musicians expanded the creative possibilities of jazz beyond the popular, dance-oriented swing style with a new "musician's music" that was not as danceable and demanded close listening.[1] As bebop was not intended for dancing, it enabled the musicians to play at faster tempos. Bebop musicians explored advanced harmonies, complex syncopation, altered chords, extended chords, chord substitutions, asymmetrical phrasing, and intricate melodies. Bebop groups used rhythm sections in a way that expanded their role. Whereas the key ensemble of the swing era was the big band of up to fourteen pieces playing in an ensemble-based style, the classic bebop group was a small combo that consisted of saxophone (alto or tenor), trumpet, piano, guitar, double bass, and drums playing music in which the ensemble played a supportive role for soloists. Rather than play heavily arranged music, bebop musicians typically played the melody of a composition (called the "head") with the accompaniment of the rhythm section, followed by a section in which each of the performers improvised a solo, then returned to the melody at the end of the composition.

Some of the most influential bebop artists, who were typically composer-performers, are: alto sax player Charlie Parker; tenor sax players Dexter Gordon, Sonny Rollins, and James Moody; clarinet player Buddy DeFranco; trumpeters Fats Navarro, Clifford Brown, Miles Davis, and Dizzy Gillespie; pianists Bud Powell, Mary Lou Williams, and Thelonious Monk; electric guitarist Charlie Christian, Joe Pass and drummers Kenny Clarke, Max Roach, and Art Blakey.

While swing music tended to feature orchestrat

While swing music tended to feature orchestrated big band arrangements, bebop music highlighted improvisation. Typically, a theme (a "head," often the main melody of a pop or jazz standard of the swing era) would be presented together at the beginning and the end of each piece, with improvisational solos based on the chords of the compositions. Thus, the majority of a piece in bebop style would be improvisation, the only threads holding the work together being the underlying harmonies played by the rhythm section. Sometimes improvisation included references to the original melody or to other well-known melodic lines ("quotes", "licks" or "riffs"). Sometimes they were entirely original, spontaneous melodies from start to finish.

Chord progressions for bebop compositions were often taken directly from popular swing-era compositions and reused with a new and more complex melody, forming new compositions (see contrafact

Chord progressions for bebop compositions were often taken directly from popular swing-era compositions and reused with a new and more complex melody, forming new compositions (see contrafact). This practice was already well-established in earlier jazz, but came to be central to the bebop style. The style made use of several relatively common chord progressions, such as blues (at base, I-IV-V, but infused with II-V motion) and 'rhythm changes' (I-VI-II-V, the chords to the 1930s pop standard "I Got Rhythm"). Late bop also moved towards extended forms that represented a departure from pop and show compositions. Bebop chord voicings often dispensed with the root and fifth tones, instead basing them on the leading intervals that defined the tonality of the chord. That opened up creative possibilities for harmonic improvisation such as tritone substitutions and use of diminished scale based improvised lines that could resolve to the key center in numerous and surprising ways.

Bebop musicians also employed several harmonic devices not typical of previous jazz. Complicated harmonic substitutions for more basic chords became commonplace. These substitutions often emphasized certain dissonant intervals such as the flat ninth, sharp ninth or the sharp eleventh/tritone. This unprecedented harmonic development which took place in bebop is often traced back to a transcendent moment experienced by Charlie Parker while performing "Cherokee" at Clark Monroe's Uptown House, New York, in early 1942. As described by Parker:[19]

I'd been getting bored with the stereotyped changes that were being used, ... and I kept thinking there's bound to be something else. I could hear it sometimes. I couldn't play it.... I was working over "Cherokee", and, as I did, I found that by using the higher intervals of a chord as a melody line and backing them with appropriately related changes, I could play the thing I'd been hearing. It came alive.

Gerhard Kubik postulates that the harmonic development in bebop sprung from the blues, and other African-related tonal sensibilities, rather than twentieth century Western art music, as some have suggested. Kubik states: "Auditory inclinations were the African legacy in [Parker's] life, reconfirmed by the experience of the blues tonal system, a sound world at odds with the Western diatonic chord categories. Bebop musicians eliminated Western-style functional harmony in their music while retaining the strong central tonality of the blues as a basis for drawing upon various African matrices."[19] Samuel Floyd states that blues were both the bedrock and propelling force of bebop, bringing about three main developments: