These words refer to forms of popular music that ultimately derive from the folk cultures of the U.S. South. Most of them share a stress on emphatic rhythm and improvisatory melodic lines. Jazz is the generic word under which nearly all the remaining terms can be grouped. In itself it can apply descriptively to any such music, regardless of period, style or the number of musicians involved: a small jazz combo; the influence of spirituals on the development of jazz ; a jazz vocalist; big-band jazz ; the new sounds in jazz . Blues and ragtime are the earliest forms of jazz ; both were evolved by American Negroes. Blues typically refers to songs whose lyrics have three-line stanzas in which the first and second line are alike. They were sung and played in a slow tempo and were in major keys (as is most jazz ), but with unconventional scale degrees (blue notes) such as flatted thirds and certain unusual harmonies. The mood expressed was of grief, melancholy or despair: the blues singing of Bessie Smith . Later, the word has been applied less exactly to any jazz or commercial music reflecting some of these qualities. Ragtime refers classically to an early form of piano jazz , originating in and around St. Louis, based on European marches and dance tunes but played with a characteristic syncopated Negro rhythm. The word came to be used loosely to refer to Negro-inspired popular music in general and thus was the predecessor to the word jazz (originally jass ): Alexander’s Ragtime Band (1911).

The Original Dixie Land Jass Band, a white group, made the New Orleans ensemble style of Negro popular music a rage in America in 1917 and established the word jazz in popular usage. Dixieland , meaning New Orleans-style jazz as played by white performers and not merely a term of geographical ascription, did not come into use until the 1940s: Dixieland at Eddie Condon’s. Swing evolved in the 1930s from the ensemble jazz of both white and Negro groups as bands became bigger and more theatrical and orchestral arrangements more polished; it puts more stress on coordinated teamwork than on improvisation: Tommy Dorsey and Benny Goodman set the tone for the age of swing . Boogie-woogie is a kind of piano jazz in which improvisations on a blues melody with the right hand are played over a repetitive eight-to-a-bar figure in the left. Swing arrangement of boogie-woogie pieces were sometimes written: a boogie-woogie number played by Count Basie’s band.

In New York City, bold Negro jazz experimentalists in the late 1940s developed re-bop, which became be-bop and finally bop . This music emphasized far-ranging improvisations on standard themes set off by closely woven ensemble work that featured pungent harmonies (such as the use of chords with a flatted fifth) and polyphonic lines. This music did not become popular in turn as swing had been, but it was adopted by musicians, both white and black, and won the interest of an intellectual and avant-garde coterie. In contrast, rhythm-and-blues (of shortened to "R and B ") have kept their hold on a mass audience of American Negroes since the "race records" of the 1920s; with their roots in the experience of rural Negroes moving to the great cities of the North, they are an authentic urban folk music, still meeting the needs of those for whom it is written.

In the 1950s rock ‘n’ roll fused elements of rhythm-and-blues , not yet generally popular, and aspects of Southern white "country music" to achieve such popularity as to sweep other forms of popular music and even of jazz , its distant relative, from public attention. This music emphasized an unvarying (even monotonous), heavy, rhythmic beat and lyrics expressive of pre-adolescent sentiments. Nearly all popular music in general, or some mixture of these. In the 1960s rock ‘n’ roll was shortened to rock to describe a later development of this music in which as few as four musicians sing and play together in close coordination; rock stresses quite conventional harmonies, a driving but less inflexible rhythmic impulse, and free-and-easy lyrics that violate accepted blues or popular song patterns: folk rock ; acid rock .