Blues Harmony

The twelve-bar blues is one of the most important harmonic forms in Western popular music. In its most basic version, it uses only three chords — the tonic (I), the subdominant (IV), and the dominant (V) — arranged in a repeating twelve-measure cycle. In the key of C: bars 1–4 are I7 (C7); bars 5–6 are IV7 (F7); bars 7–8 return to I7; bar 9 is V7 (G7); bar 10 is IV7; bars 11–12 are I7 with a turnaround. This skeleton underlies everything from early Delta blues to Charlie Parker's bebop compositions.

A striking feature of blues harmony is that all three chords are dominant seventh chords — major triads with minor sevenths — even the tonic and subdominant. In classical harmony a tonic chord with a minor seventh would demand resolution; in the blues, it is simply a tonal color, part of the genre's characteristic sound. This use of unresolved dominant sevenths reflects the African-American musical tradition from which the blues emerged and sets it apart from European tonal practice.

Jazz musicians elaborate the basic twelve-bar form with chord substitutions and additional changes called the "jazz blues." A typical jazz blues in C might replace bars 3–4 with a passing move to IV (sometimes called a "quick change"), add ii–V–I motions leading into bars 5 and 9, and end with a ii–V turnaround in bars 11–12. The bebop blues adds even more substitutions: a tritone sub at bar 9, secondary dominants, and diminished passing chords, producing a richly chromatic version of the same twelve-bar skeleton.

The turnaround is the final two bars of each chorus, designed to propel the music back to the beginning. A simple turnaround is I7–V7; a jazz turnaround might be Imaj7–VIm7–iim7–V7, or an even more embellished chain of dominant seventh chords cycling around the circle of fifths. The turnaround signals the end of one chorus and the beginning of the next, allowing musicians to loop the form indefinitely in a jam or solo setting.