Bebop or bop is a style of jazz developed in the early to mid-1940s in the United States, which features songs 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. As bebop was not intended for dancing, it enabled the musicians to play at faster tempos. Tenor sax players Dexter Gordon, Sonny Rollins, and James Moody; alto sax player Charlie Parker; trumpeters Fats Navarro, Clifford Brown, and Dizzy Gillespie; pianists Bud Powell, Mary Lou Williams, and Thelonious Monk; electric guitarist Charlie Christian, and drummers Kenny Clarke, Max Roach, and Art Blakey were some of the most influence bebop artists. Bebop grew out of the culmination of trends that had been occurring within swing music since the mid-1930s. One young admirer of the Basie orchestra in Kansas City was a teenage alto saxophone player named Charlie Parker. He was especially enthralled by their tenor saxophone player Lester Young, who played long flowing melodic lines that wove in and out of the chordal structure of the tune but somehow always made musical sense. As the 1930s turned to the 1940s, Parker went to New York as a featured player in the Jay McShann Orchestra. In New York he found other musicians who were exploring the harmonic and melodic limits of their music, including Dizzy Gillespie, a Roy Eldridge-influenced trumpet player who, like Parker, was exploring ideas based on upper chord intervals, beyond the sevenths that had traditionally defined jazz harmony. Drummers such as Kenny Clarke and Max Roach were extending the path set by Jo Jones, adding the ride cymbal to the high hat cymbal as a primary timekeeper and reserving the bass drum for accents. Bass drum accents were colloquially termed "bombs," which referenced events in the world outside of New York as the new music was being developed. The new style of drumming supported and responded to soloists with accents and fills, almost like a shifting call and response. This change increased the importance of the string bass. Bebop originated as "musicians' music", played by musicians with other money-making gigs who did not care about the commercial potential of the new music. It did not attract the attention of major record labels nor was it intended to. Some of the early bebop was recorded informally. Some sessions at Minton's in 1941 were recorded, with Thelonious Monk alongside an assortment of musicians including Joe Guy, Hot Lips Page, Roy Eldridge, Don Byas, and Charlie Christian. Christian is featured in recordings from May 12, 1941 (Esoteric ES 548). Charlie Parker and Dizzy Gillespie were both participants at a recorded jam session hosted by Billy Eckstine on February 15, 1943, and Parker at another Eckstine jam session on February 28, 1943 (Stash ST-260; ST-CD-535). By 1946 bebop was established as a broad-based movement among New York jazz musicians, including trumpeters Fats Navarro and Kenny Dorham, trombonists J. J. Johnson and Kai Winding, alto saxophonist Sonny Stitt, tenor saxophonists Lucky Thompson and James Moody, baritone saxophonists Leo Parker and Serge Chaloff, vibraphonist Milt Jackson, pianists Erroll Garner and Al Haig, bassist Slam Stewart, and others who would contribute to what would become known as "modern jazz". The new music was gaining radio exposure with broadcasts such as those hosted by "Symphony Sid" Torin. Bebop was taking root in Los Angeles as well, among such modernists as trumpeters Howard McGhee and Art Farmer, alto players Sonny Criss and Frank Morgan, tenor player Teddy Edwards, trombonist Melba Liston, pianists Dodo Marmarosa, Jimmy Bunn and Hampton Hawes, guitarist Barney Kessel, bassists Charles Mingus and Red Callender, and drummers Roy Porter and Connie Kay. Gillespie's "Rebop Six" (with Parker on alto, Lucky Thompson on tenor, Al Haig on piano, Milt Jackson on vibes, Ray Brown on bass, and Stan Levey on drums) came to Los Angeles in December 1945. Parker remained in Los Angeles after the rest of the band left, performing and recording for six months. Parker's tenure in Los Angeles, the arrival of Dexter Gordon and Wardell Gray later in the year, and the promotional efforts of Ross Russell, Norman Granz, and Gene Norman helped solidify the city's status as a center of the new music. By 1950, bebop musicians such as Clifford Brown and Sonny Stitt 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. The early 1950s also saw some smoothing in Charlie Parker's style. 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. classic bebop combo consisted of saxophone, trumpet, double bass, drums and piano. This was a format used (and popularized) by both Parker (alto sax) and Gillespie (trumpet) in their 1940s groups and recordings, sometimes augmented by an extra saxophonist or guitar (electric or acoustic), occasionally adding other horns (often a trombone) or other strings (usually violin) or dropping an instrument and leaving only a quartet.The musical devices developed with bebop were influential far beyond the bebop movement itself. "Progressive Jazz" was a broad category of music that included bebop-influenced "art music" arrangements used by big bands such as those led by Boyd Raeburn, Charlie Ventura, Claude Thornhill, and Stan Kenton, and the cerebral harmonic explorations of smaller groups such as those led by pianists Lennie Tristano and Dave Brubeck. Voicing experiments based on bebop harmonic devices were used by Miles Davis and Gil Evans for the groundbreaking "Birth of the Cool" sessions in 1949. Musicians who followed the stylistic doors opened by Davis, Evans, Tristano, and Brubeck would form the core of the cool jazz and "west coast jazz" movements of the early 1950s.