Twelve-bar Blues - Analysis

Analysis

The twelve-bar blues, a chromatic chord progression, is a logical formula for blues music: without the dominant's major minor seventh chord (in C: G7), the sequence does not accord with the tonal "V-I" relationship. Instead, it would be based mostly on a plagal cadence — an I-IV change (in C: C-F). The key is fully verified with the V7 (G7) chord, but only after going over the subdominant (F) and tonic (C).

Additionally, the chord progression meshes elements of major and minor. The major-minor (dominant) seventh chords used on each degree alone seem to fall in some grey area between the strong, content major chord and the somber, conflicted minor chord. The subdominant's seventh chord is of note here, because of its odd relationship with the tonic.

Were this classical music being discussed, the dominant (major-minor) seventh chord on the tonic would almost certainly resolve elsewhere (rather than being resolved to), especially its subdominant (from C7: to F). While, at first, it seems to resolve well to the subdominant, this is merely a tonicization (brief leave to another key), because of the earlier emphasis on the dominant seventh (C7), and because of the dominant seventh that appears on the subdominant, an element found in the Dorian mode. Traditionally, the seventh of the subdominant chord would not be flattened, as it would contradict the third of the tonic chord. This undermines the expected resolution and also questions whether the actual tonic is major or minor in quality: this seventh chord (F-A-C-E♭) resolves back to the tonic by resolving both up a step to (E♭-->E) (mediant), and down a step to from (F-->E) (leading tone); and down harmonically to I.

When returning to the I7 chord, the major third sounds like a Picardy third resolution, and the minor seventh no longer seems to resolve to the sixth (B♭-->A, the third of IV; instead it seems like a blue note that adds a tense, funky, thick color to the tonic.