With its roots in the blues, jazz has been referred to as America's classical music, yet has also become a major global phenomenon, branching off into a variety of forms. Earlier pioneers like Scott Joplin and Jelly Roll Morton paved the way for the swinging big-band sounds of Louis Armstrong and Duke Ellington. In contrast, contemporaries Dizzie Gillespie, Charlie Parker and Thelonious Monk developed bebop, with its speedy, dissonant harmonies and improvisations. And Miles Davis heralded the birth of cool jazz, modal jazz and fusion at different points in his career. Famous jazz instrumentalists have tended to be male, yet women have been at the forefront of the genre when it comes to vocalization, from the brassy blues of Bessie Smith to the haunting eclecticism of Nina Simone.