In the 1940s, the younger generation of jazz musicians created a new style that came out of the 1930s' swing music. They partially strived to counter the popularization of swing with non-danceable music that demanded listening. 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. While Gillespie was with Cab Calloway, he practiced with bassist Milt Hinton and developed some of the key harmonic and chordal innovations that would be the cornerstones of the new music; Charlie Parker did the same with bassist Gene Ramey while with McShann's group. These forerunners of the new music (which would later be termed bebop or bop—although Parker himself never used the term, feeling it demeaned the music) began exploring advanced harmonies, complex syncopation, altered chords, and chord substitutions. The bop musicians advanced these techniques with a more freewheeling, intricate and often arcane approach.
Minton's Playhouse in New York served as an incubator and experimental theater for early bebop players, including Don Byas, Thelonious Monk, and Charlie Christian, who had already hinted at the bop style in innovative solos with Benny Goodman's band. Part of the atmosphere created at jams like the ones found at Minton's Playhouse was an air of exclusivity: the "regular" musicians would often reharmonize the standards in order to exclude those whom they considered outsiders or simply weaker players.
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.