Chord rewrite rules

File:Twelve bar boogie-woogie blues in C.png in red {{audio|Twelve bar boogie-woogie blues in C.mid|Play}}.]]

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

Steedman (1984)Steedman M.J., "A Generative Grammar for Jazz Chord Sequences", Music Perception 2 (1) (1984) 52–77. 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

{{clear}}

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

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

{{clear}}

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

• use of chromatic passing chords:

...7 8 9 ...

...III⁷ / {{music|flat}}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":

{{clear}}

1 2 3 4 5 6 7 8 9 10 11 12

{{music|♭}}VI, {{music|♭}}III / {{music|♭}}VII, IV / I / I // {{music|♭}}VI, {{music|♭}}III / {{music|♭}}VII, IV / I / I // {{music|♭}}VI, {{music|♭}}III / {{music|♭}}VII, IV / I / I //

These often result in Aeolian harmony and lack perfect cadences (V–I). Middleton (1990)Middleton, Richard (1990). Studying Popular Music, p.198. {{ISBN|0-335-15275-9}}. 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 {{music|♭}} notation, see Borrowed chord.