Chord rewrite rules

File:Twelve bar boogie-woogie blues in C.png

Typical boogie woogie bassline on 12 bar blues progression in C, chord roots in red .

In music, a rewrite rule is a recursive generative grammar, which creates a chord progression from another. Steedman (1984) 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

1 2 3 4 5 6 7 8 9 10 11 12 I / I / I / I // IV /IV / I / I // V / IV / I / I where the top line numbers each bar, one slash indicates a bar line, two indicate both a bar line and a phrase ending and a Roman numeral indicates the chord function. Important transformations include

File:Chord rewrite rules I.png

Chord rewrite rules I: replacement or substitution of a chord by its dominant or subdominant .

• replacement or substitution of a chord by its dominant or subdominant:

1 2 3 4 5 6 7 8 9 10 11 12 I / IV / I / I⁷ // IV / VII⁷ / III⁷ / VI⁷ // II⁷ / V⁷ / I / I //

File:Chord rewrite rules II.png

Chord rewrite rules II: use of chromatic passing chords .

• use of chromatic passing chords:

...7 8 9 ... ...III⁷ / ♭III⁷ / II⁷...

• and chord alterations such as minor chords, diminished sevenths, etc.

Sequences by fourth, rather than fifth, include Jimi Hendrix's version of "Hey Joe" and Deep Purple's "Hush":

1 2 3 4 5 6 7 8 9 10 11 12 ♭VI, ♭III / ♭VII, IV / I / I // ♭VI, ♭III / ♭VII, IV / I / I // ♭VI, ♭III / ♭VII, IV / I / I // These often result in Aeolian harmony and lack perfect cadences (V–I). Middleton (1990) suggests that both modal and fourth-oriented structures, rather than being, "distortions or surface transformations of Schenker's favoured V-I kernel, are more likely branches of a deeper principle, that of tonic/not-tonic differentiation." For the ♭ notation, see Borrowed chord.