Bebop or bop is a form of jazz characterized by fast tempos and improvisation based on harmonic structure rather than melody. It was developed in the early and mid-1940s. It first surfaced in musicians' argot some time during the first two years of the Second World War.
By the mid-1950s musicians (Miles Davis and John Coltrane among others) began to explore directions beyond the standard bebop vocabulary. Simultaneously, other players expanded on the bold steps of bebop: "cool jazz" or "West Coast jazz", modal jazz, as well as free jazz and avant-garde forms of development from the likes of George Russell.