Jazz influenced many performers of all the major styles of later popular music, though jazz itself never again became such a major part of American popular music as during the swing era. The later 20th-century American jazz scene did, however, produce some popular crossover stars, such as Miles Davis. In the middle of the 20th century, jazz evolved into a variety of subgenres, beginning with bebop. Bebop is a form of jazz characterized by fast tempos, improvisation based on harmonic structure rather than melody, and use of the flatted fifth. Bebop was developed in the early and mid-1940s, later evolving into styles like hard bop and free jazz. Innovators of the style included Charlie Parker and Dizzy Gillespie, who arose from small jazz clubs in New York City.