The blues is a special category of black folk song whose subject is loneliness, trouble, and unhappiness of every shade. Indeed, the blues is more than song, more than music: It is an essential expression of the African American experience. Though gloom and dejection are at the heart of the blues, not infrequently blues lyrics also convey humor, banter, and especially hope and resilience.

Emerging around 1900, the blues was a major influence on early jazz — and has remained a major force in American music ever since.

Like most folk songs, blues are strophic songs, with many stanzas sung to the same melody, as the singer develops his or her thought, often on the spur of the moment. A blues melody consists typically of three four-measure phrases — hence the expression twelve-bar blues — while the matching stanza is in a poetic a a b form (line 1 repeated before line 3, which is a miniature punch line). Here are stanzas 1 and 4 of “If You Ever Been Down” Blues:

Composed blues — for example, W. C. Handy’s famous “St. Louis Blues” — can be more complicated than this one, but the a a b poetic scheme is basic for the blues.

Blues melodies (and the bass lines and harmonies under blues melodies) provided jazz musicians with powerfully emotional patterns for improvisation. But more than that, blues also provided jazz with a sonorous model. Jazz instrumental playing has an astonishing vocal quality, as though in imitation of the blues. The trumpet, saxophone, and trombone sound infinitely more flexible and “human” played in jazz style than when played in military band or symphonic style. Jazz instruments seem to have absorbed the vibrant accents of black singing. (This is a feature that jazz passed on to rock music, where the electric guitar is the instrument that powerfully imitates the voice.)