The 12-bar jazz blues

The blues is the most played form in jazz. Its basic structure fits into 12 bars and three chords, all dominant sevenths (a peculiarity of the blues: even the I chord is a “7” chord).

The basic blues (in F)

Three four-bar phrases: statement (I), answer on IV, conclusion (V – IV – I). In sung blues, the lyrics often follow an AAB pattern: the first line is repeated, then answered.

The jazz blues

Jazz musicians enrich this chart with ii–Vs and passing chords:

• Bar 2: a quick visit to IV (the “quick change”).
• Bar 4: a ii–V into B♭7 (Cm7 – F7 = ii–V of B♭).
• Bar 6: B°7, a passing diminished chord that walks the bass up B♭ → B → C (F7/C).
• Bars 8-10: D7 (V/ii) then the ii–V Gm7 – C7 leading back to F.
• Bars 11-12: the turnaround I – VI7 – ii – V that sets the chart going again.

The first four bars, with shells in the left hand and a blues phrase in the right:

M:4/4
L:1/8
%%staves {RH LH}
V:RH clef=treble
V:LH clef=bass
K:F
[V:RH] "F7"F _A =A c z4 | "Bb7"_e d c _A F2 z2 | "F7"F _A =A c d c _A F | "Cm7"B2 G2 "F7"A2 c2 |]
[V:LH] [F,A,_E]8 | [B,,_A,D]8 | [F,A,_E]8 | [C,_E,B,]4 [F,,_E,A,]4 |]

Notice the A♭ sliding up to A♮: that's the blue note, the minor third played over a major chord, the expressive heart of the blues.