Rhythm and Blues was the urban popular black music of the 1940s and 1950s. Its antecedents were the jazz and blues of the 1930s, especially Kansas City jazz; in the 1960s, it turned into soul. R&B, as it is often known, was the precursor and the vital center of rock 'n' roll. It used small-group jazz instrumentation, centered on piano and saxophone as often as on guitar, and it moved in the direction of straightforward, danceable rhythms at the time when jazz was moving toward the more complex structures of bebop. Blending the emotional immediacy of the blues, the instrumental intensity of jazz, and the wit of black vaudeville, it became arguably the most irresistible of American musical forms.

Billboard magazine first used the term "Rhythm and Blues" as the title for its black music charts in 1949, replacing "race music." But more than...