Blues Style

During the first decades of the twentieth century blues music was not clearly defined in terms of a chord progression. There were many blues in 8-bar form, such as "How Long Blues", "Trouble in Mind", and Big Bill Broonzy's "Key to the Highway". Idiosyncratic numbers of bars are also encountered occasionally, as with the 9-bar progression in Howlin' Wolf's "Sitting on Top of the World". The basic twelve-bar lyric framework of a blues composition is reflected by a standard harmonic progression of twelve bars, in 4/4 or (rarely) 2/4 time. Slow blues are often played in 12/8 (4 beats per measure with 3 subdivisions per beat), that is, 4/4 time with triplets.

By the 1930s, twelve-bar blues became the standard. There would also be 16 bar blues, as in Ray Charles's instrumental "Sweet 16 Bars", and in Herbie Hancock's "Watermelon Man". The blues chords associated to a twelve-bar blues are typically a set of three different chords played over a twelve-bar scheme:

I	I or IV	I	I7
IV	IV	I	I7
V	IV	I	I or V

where the Roman numbers refer to the degrees of the progression. That would mean, if played in the tonality of C, the chords would be as follows:

C	C or F	C	C
F	F	C	C
G	F	C	C or G

(When the IV chord is played in bar 2, the blues is called a "Quick-Change" blues).

In this example, C is the tonic chord, F the subdominant. Much of the time, some or all of these chords are played in the harmonic seventh (7th) form. Frequently, the last chord is the dominant (V or in this case G) turnaround making the transition to the beginning of the next progression.

The use of the harmonic seventh interval is a characteristic of blues, and is popularly called the "blues seven" . At a 7:4 ratio, it is not close to any interval, minor or major, on the conventional Western diatonic scale . However, through convenience or necessity it is often approximated by a minor seventh interval, or in terms of chords, a dominant seventh chord.