The 12-bar blues is the most popular and the most-used blues form.
Here is the basic 12-bar form with Roman numerals representing the scale degree of the (major) chord. This can be applied in any key:
I I I I
IV IV I I
V IV I I
So in the key of C (all white keys on the piano), these chords would be used:
C C C C
F F C C
G F C C
These chords would be used in the key of E:
E E E E
A A E E
B A E E
Each numeral or letter represents one bar (measure) of four beats. The beats per minute, or tempo, can vary. A credible tempo for demonstration purposes could be two beats per second or 120 beats per minute.
A typical rhythm in performing the blues is a shuffle. This applies a triplet or a triplet feel to each beat.
Most blues performers add a dominant seventh (flatted seventh):
C7 C7 C7 C7
F7 F7 C7 C7
G7 F7 C7 C7
C7 chord = notes C E G Bb
F7 chord = notes F A C Eb
G7 chord = notes G B D F
This video demonstrates the basic 12-bar blues form:
Click here to see the entire video with with more information:
Complete Video
Adding the dominant seventh comports with the blues being based on the pentatonic (5-note) scale. In the key of C, this would be: C-Eb-F-G-Bb. Blues melodies also use mainly these notes and will often add the flatted fifth (Gb in the key of C).
The E-natural in the C chord is outside the strict pentatonic scale, but it seems to work since the pentatonic blues pitches aren’t exact in our Western tempered tuning.
Singers and guitarists will often “bend” notes, adding to the blues’ characteristic flavor.
Another common variation is to insert the IV chord in the second measure; and/or insert a V chord (a “turnaround” to lead back to the beginning) in the 12th measure:
C7 F7 C7 C7
F7 F7 C7 C7
G7 F7 C7 G7
Another variation sometimes used in a jazz performance:
C7 F7 C7 C7
F7 F7 C7 B7 Bb7 A7
D7 G7 C7 A7 D7 G7
Another jazz variation:
C Bm7-5 E7b9 Am7 D7 Gm7 C7
F7 Fm7 Bb7 Em7 A7 Ebm7 Ab7
Dm7 G7 C Am7 Dm7 G7
Typical blues pattern in a minor key (Cm)::
Cm Cm Cm Cm
Fm Fm Cm Cm
Ab7 G7 Cm Cm