The basic form
Twelve bars in three phrases of four. In C:
| C7 | C7 | C7 | C7 |
|---|---|---|---|
| F7 | F7 | C7 | C7 |
| G7 | F7 | C7 | G7 |
The last bar (turnaround) leads back to the beginning. Variants: quick change (F7 in bar 2), turnaround C7 – A7 – Dm7 – G7.
“Non-classical” harmony
All three chords are dominant sevenths, but they don't have a dominant function: they are colors. The C7 doesn't want to go to F, it stays on it for four bars. It is one of the points where the blues breaks the rules of classical tonality.
Blue notes
The melody uses the b3 (Eb), the b5 (Gb) and the b7 (Bb) over major chords: the friction between the melody's Eb and the chord's E is the sound of the blues.
Jazz blues and minor blues
- Jazz blues: C7 | F7 | C7 | Gm7 C7 | F7 | F#°7 | C7 | Em7 A7 | Dm7 | G7 | C7 A7 | Dm7 G7 |.
- Minor blues in A: | Am7 | Am7 | Am7 | Am7 | Dm7 | Dm7 | Am7 | Am7 | F7 | E7 | Am7 | E7 |, with the F7 (bVI7) falling by semitone to E7 (V7).
Lyric form
Traditionally AAB: a line, repeated, then a response. Often with call and response between voice and instrument.
Influence
The blues is the root of rock, R&B, funk, hip hop: the minor pentatonic riff, the dominant seventh as color, the shuffle groove.
Real example
“Johnny B. Goode” (Chuck Berry, 1958) is a 12-bar blues in almost pure form; listening to the guitar filling the spaces of the voice is a lesson in call and response.