Jazz characterized by harmonic complexity, convoluted melodic lines, and frequent shifting of rhythmic accent. In the mid-1940s, a group of musicians, including Dizzy Gillespie, Thelonious Monk, and Charlie Parker, rejected the conventions of swing to pioneer a self-consciously artistic extension of improvised jazz, which set new technical standards of velocity and harmonic subtlety. Two genres grew out of bebop in the 1950s: the delicate, dry, understated approach that came to be known as cool jazz, and the aggressive, blues-tinged earthiness of hard bop.
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In the 1940s, the younger generation of jazz musicians forged a new style out of the swing music of the 1930s. Mavericks like Dizzy Gillespie, Charlie Parker, Bud Powell and Thelonious Monk were influenced by the preceding generation's adventurous soloists, such as pianists Art Tatum and Earl Hines, tenor saxophonists Coleman Hawkins and Lester Young and trumpeter Roy Eldridge. Gillespie and Parker, both out of the Earl Hines Band in Chicago had traveled with some of the pre-bop masters, including Jack Teagarden, Earl Hines and Jay McShann. These forerunners of bebop began exploring advanced harmonies, complex syncopation, altered chords, and chord substitutions and the bop generation advanced these techniques with a more freewheeling and often arcane approach.
Minton's Playhouse in New York served as a workout room and experimental theater for early bebop players, including Charlie Christian, who had already hinted at the bop style in innovative solos with Benny Goodman's band.
Christian's major influence was in the realm of rhythmic phrasing. Christian commonly emphasized weak beats and off beats, and often ended his phrases on the second half of the fourth beat. Christian experimented with asymmetrical phrasing, which was to become a core element of the new bop style. Swing improvisation was commonly constructed in two or four bar phrases that corresponded to the harmonic cadences of the underlying song form. Bop improvisers would often deploy phrases over an odd number of bars, and overlap their phrases across bar lines and across major harmonic cadences. Christian and the other early boppers would also begin stating a harmony in their improvised line before it appeared in the song form being outlined by the rhythm section. This momentary dissonance creates a strong sense of forward motion in the improvisation. Swing improvisers commonly emphasized the first and third beats of a measure. But in a bebop composition such as Dizzy Gillespie's "Salt Peanuts", the rhythmic emphasis switches to the second and fourth beats of the measure. Such new rhythmic phrasing techniques give the typical bop solo a feeling of floating free over the underlying song form, rather than being tied into the song form.
Swing drummers had kept up a steady four-to-the-bar pulse on the bass drum. Bop drummers, led by Kenny Clarke, moved the drumset's time-keeping function to the ride or hi-hat cymbal, reserving the bass drum for accents. Bass drum accents were colloquially termed "dropping bombs." Notable bop drummers such as Max Roach, Philly Joe Jones, Roy Haynes, and Kenny Clarke began to support and respond to soloists, almost like a shifting call and response.
This change increased the importance of the string bass. Now, the bass not only maintained the music's harmonic foundation, but also became responsible for establishing a metronomic rhythmic foundation by playing a "walking" bass line of four quarter notes to the bar. While small swing ensembles commonly functioned without a bassist, the new bop style required a bass in every small ensemble.
By 1950, a second wave of bebop musicians — such as Clifford Brown, Sonny Stitt, and Fats Navarro — began to smooth out the rhythmic eccentricities of early bebop. Instead of using jagged phrasing to create rhythmic interest, as the early boppers had, these musicians constructed their improvised lines out of long strings of eighth notes, and simply accented certain notes in the line to create rhythmic variety.
Bebop differed drastically from the straightforward compositions of the swing era, and was instead characterized by fast tempos, asymmetrical phrasing, intricate melodies, and rhythm sections that expanded on their role as tempo-keepers. The music itself seemed jarringly different to the ears of the public, who were used to the bouncy, organized, danceable tunes of Benny Goodman and Glenn Miller during the swing era. Instead, bebop appeared to sound racing, nervous, and often fragmented. But to jazz musicians and jazz music lovers, bebop was an exciting and beautiful revolution in the art of jazz.
While swing music tended to feature orchestrated big band arrangements, bebop music was much more free in its structure. 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 the chords of the tune. Thus, the majority of a song 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 ("allusions," or "riffs"). Sometimes they were entirely original, spontaneous melodies from start to finish.
Chord progressions for bebop tunes were often taken directly from popular swing-era songs and reused with a new and more complex melody, forming new compositions. This practice was already well-established in earlier jazz, but it came to be central to the bebop style.
Although only one part of a rich jazz tradition, bebop music continues to be played regularly throughout the world. Trends in improvisation since its era have changed from its harmonically-tethered style, but the capacity to improvise over a complex sequence of altered chords is a fundamental part of any jazz education.
Bebop style also influenced the Beat Generation whose spoken-word style drew on jazz rhythms, and whose poets often employed jazz musicians to accompany them. The bebop influence also shows in rock and roll, which contains solos employing a form similar to bop solos, and "hippies" of the 60s and 70s, who, like the boppers had a unique, non-conformist style of dress, a vocabulary incoherent to outsiders, and a communion through music. Fans of bebop were not restricted to the USA; the music gained cult status in France and Japan.
More recently, Hip-hop artists (A Tribe Called Quest, Guru) have cited bebop as an influence on their rapping and rhythmic style. Bassist Ron Carter even collaborated with A Tribe Called Quest on 1991's The Low End Theory, and vibraphonist Roy Ayers and trumpeter Donald Byrd were featured on Jazzmatazz, by Guru, in the same year. Bebop samples, especially bass lines, ride cymbal swing clips, and horn and piano riffs are found throughout the hip-hop compendium.