Chord rewrite rules

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 original progression may be notated as follows (typical 12-bar blues):

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 numbers on the top line indicate each bar, one slash indicating a bar line and two indicating a phrase marking, and the Roman numerals indicating the chord function. Important transformations include

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//

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.