Blues

Generally a 12 or 24 bar progression of music in which the first 1/3 of the progression is mostly focused on the tonic or I chord, the second 1/3 usually split between the subdominant and tonic or IV and I chords, and the final 1/3 resolved from the dominant to the tonic, or V and I chords. A 16 bar blues is possible, usually by doubling the first four bars. Origins vary in terms of location and time period, but largely thought to developed in the American South between the 1880s and 1910s. It is the basis of much improvised traditional and modern jazz, and of both the Rhythm and Blues and Rock and Roll formats of the 1930s to 1950s.