Many musical styles flourished and combined in the 1940s and 1950s, most likely because of the influence of radio had in creating a mass market for music. World War II caused great social upheaval, and the music of this period shows the effects of that upheaval.
In the 1940s, the major strands of American music combined to form what would eventually be coined as rock and roll. Based most strongly off an electric guitar-based version of the Chicago blues, rock also incorporated jazz, country, folk, swing, and other types of music; in particular, bebop jazz and boogie woogie blues were in vogue and greatly influenced the music style. The style had developed by 1949, and quickly became popular among blacks nationwide (see 1949 in music). Mainstream success was slow to develop, though (in spite of early success with Bill Haley & His Comets' "Rock Around the Clock"), and didn't begin in earnest until Elvis Presley ("Hound Dog") began singing rock, R&B and rockabilly songs in a devoted black style. He quickly became the most famous and best-selling artist in American history, and a watershed point in the development of music.