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Seventh chords are a type of chord that includes the 7th scale degree (that is, the 7th note of the scale). There are different types of 7th chords such as major 7ths, dominant 7ths, minor 7ths, half diminished 7ths, and fully diminished 7ths. [8] These chords are similar with slight changes, but are all centered around the same key center.
At its most basic, a single version of this blues scale is commonly used over all changes (or chords) in a twelve-bar blues progression. [7] Likewise, in contemporary jazz theory, its use is commonly based upon the key rather than the individual chord. [2] Greenblatt defines two blues scales, the major and the minor.
[36] [37] For example, a twelve-bar blues progression of chords in the key of E has three sets of four bars: E–E–E–E7 A–A–E–E B7–A–E–B7; this progression is simplified by playing the sevenths as major chords. [36] The twelve-bar blues structure is used by McCartney's "3 Legs", [32] which was noted earlier.
A common type of three-chord song is the simple twelve-bar blues used in blues and rock and roll. Typically, the three chords used are the chords on the tonic, subdominant, and dominant (scale degrees I, IV and V): in the key of C, these would be the C, F and G chords. Sometimes the V 7 chord is used instead of V, for greater tension.
Since it is the dominant chord a tritone away, the substitute dominant may resolve down a fifth, to a tonic chord a tritone away from the previous tonic (for example, in F one may feature a ii–V on C, which with a substitute dominant resolves to G ♭, a distant key from F). Resolution from the substituted chord to the original tonic is also ...
Steedman (1984) [1] has proposed a set of recursive "rewrite rules" which generate all well-formed transformations of jazz, basic I–IV–I–V–I twelve-bar blues chord sequences, and, slightly modified, non-twelve-bar blues I–IV–V sequences ("rhythm changes"). The typical 12-bar blues progression can be notated
The blues' 12-bar structure and the blues scale was a major influence on rock and roll music. Rock and roll has been called "blues with a backbeat"; Carl Perkins called rockabilly "blues with a country beat". Rockabillies were also said to be 12-bar blues played with a bluegrass beat.
In rock and blues, musicians also often refer to chord progressions using Roman numerals, as this facilitates transposing a song to a new key. For example, rock and blues musicians often think of the 12-bar blues as consisting of I, IV, and V chords. Thus, a simple version of the 12-bar blues might be expressed as I–I–I–I, IV–IV–I–I ...