Jazz has been called America's classical music, and for good reason. Along with the blues, its forefather, it is one of the first truly indigenous musics to develop in America, yet its unpredictable, risky ventures into improvisation gave it critical cache with scholars that the blues lacked. At the outset, jazz was dance music, performed by swinging big bands. Soon, the dance elements faded into the background and improvisation became the key element of the music. As the genre evolved, the music split into a number of different styles, from the speedy, hard-hitting rhythms of be-bop and the laid-back, mellow harmonies of cool jazz to the jittery, atonal forays of free jazz and the earthy grooves of soul jazz. What tied it all together was a foundation in the blues, a reliance on group interplay and unpredictable improvisation. Throughout the years, and in all the different styles, those are the qualities that defined jazz. --All Music Guide