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Musical 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". There are also 16 bar blues, as in Ray Charles's instrumental "Sweet 16 Bars", and in Herbie Hancock's "Watermelon Man". More 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).

By the 1930s, twelve-bar blues became more standard. 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	I
IV	IV	I	I
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 F, the chords would be as follows:

F	F or Bb	F	F
Bb	Bb	F	F
C	Bb	F	F or C

(When the IV chord is played in bar 2, the blues is called a "Quick-Change" blues). In this example, F is the tonic chord, Bb the subdominant. Note that much of the time, every chord is played in the dominant seventh (7th) form. Frequently, the last chord is the dominant (V or in this case C) turnaround making the transition to the beginning of the next progression.

Minor pentatonic blues scale on A

The lyrics generally end on the last beat of the tenth bar or the first beat of the eleventh bar, and the final two bars are given to the instrumentalist as a break; the harmony of this two-bar break, the turnaround, can be extremely complex, sometimes consisting of single notes that defy analysis in terms of chords. The final beat, however, is almost always strongly grounded in the dominant seventh (V7), to provide tension for the next verse.

Sheet music from "St. Louis Blues" (1914)

Melodically, blues music is marked by the use of the flatted third, fifth and seventh (the so-called blue or bent notes) of the associated major scale. These scale tones can replace the natural scale tones or be added to the scale, as in the case of the minor pentatonic blues scale, where the flatted third replaces the natural third, the flatted seventh replaces the natural seventh and the flatted fifth is added in between the natural fourth and natural fifth. While the twelve-bar harmonic progression had been intermittently used for centuries, the revolutionary aspect of blues was the frequent use of the flatted third, flatted seventh, and even flatted fifth in the melody, together with crushing—playing directly adjacent notes at the same time, i.e., diminished second—and sliding—similar to using grace notes.

Whereas a classical musician will generally play a grace note distinctly, a blues singer or harmonica player will glissando, "crushing" the two notes and then releasing the grace note. In blues chord progressions, the tonic, subdominant and dominant chords are often played as dominant sevenths, the lowered seventh (minor seventh) being an important component of the blues scale. Blues is also occasionally played in a minor key. The scale differs little from the traditional minor, except for the occasional use of a flatted fifth in the tonic, often sang or played by the singer or lead instrument with the perfect fifth in the harmony.

• Janis Joplin's rendition of "Ball and Chain", accompanied by Big Brother and the Holding Company, provides an example of this technique.
• Minor-key blues is often structured in sixteen bars rather than twelve, in the style of gospel music, as in "St. James Infirmary Blues" and Trixie Smith's "My Man Rocks Me."
• Sometimes, a Dorian scale is used for minor-key blues, with its minor third and seventh but major sixth.

Blues shuffles reinforce the trance-like rhythm and call-and-response, and form a repetitive effect called a " groove". The simplest shuffles commonly used in many postwar electric blues, rock-and-rolls, or early bebops were a three-note riff on the bass strings of the guitar. When this riff was played over the bass and the drums, the groove "feel" is created. The walking bass is another device that helps to create a "groove" . The last bar of the chord progression is usually accompanied by a turnaround that makes the transition to the beginning of the next progression.

Shuffle rhythm is often vocalized as "dow, da dow, da dow, da" or "dump, da dump, da dump, da" as it consists of uneven, or "swung", eighth notes. On a guitar this may be done as a simple steady bass or may add to that stepwise quarter note motion from the fifth to the sixth of the chord and back. An example is provided by the following tablature for the first four bars of a blues progression in E:

   E7                  A7                  E7                  E7
E |-------------------|-------------------|-------------------|-------------------|
B |-------------------|-------------------|-------------------|-------------------|
G |-------------------|-------------------|-------------------|-------------------|
D |-------------------|2--2-4--4-2--2-4--4|-------------------|-------------------|
A |2--2-4--4-2--2-4--4|0--0-0--0-0--0-0--0|2--2-4--4-2--2-4--4|2--2-4--4-2--2-4--4|
E |0--0-0--0-0--0-0--0|-------------------|0--0-0--0-0--0-0--0|0--0-0--0-0--0-0--0|