View source for Chord rewrite rules

[[File:Twelve bar boogie-woogie blues in C.png|thumb|right|350px|Typical boogie woogie bassline on 12 bar blues progression in C, chord [[root (chord)|roots]] in red {{audio|Twelve bar boogie-woogie blues in C.mid|Play}}.]]
In music, a '''rewrite rule''' is a [[recursion|recursive]] [[generative grammar]], which creates a [[chord progression]] from another.
Steedman (1984)<ref>Steedman M.J., "A Generative Grammar for Jazz Chord Sequences", ''Music Perception'' '''2''' (1) (1984) 52–77.</ref> has proposed a set of recursive "rewrite rules" which generate all [[well-formed scale|well-formed]] [[Transformation (music)|transformation]]s 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|thumb|right|350px|Chord rewrite rules I: replacement or substitution of a chord by its dominant or subdominant {{audio|Chord rewrite rules I.mid|Play}}.]]
*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 / I<sup>7</sup> // IV / VII<sup>7</sup> / III<sup>7</sup> / VI<sup>7</sup> // II<sup>7</sup> / V<sup>7</sup> / I / I //
[[File:Chord rewrite rules II.png|thumb|right|350px|Chord rewrite rules II: use of chromatic passing chords {{audio|Chord rewrite rules II.mid|Play}}.]]
*use of chromatic [[passing chord]]s:
...7      8     9 ...
...III<sup>7</sup> / {{music|flat}}III<sup>7</sup> / II<sup>7</sup>...
*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 cadence]]s (V–I). Middleton (1990)<ref>[[Richard Middleton (musicologist)|Middleton, Richard]] (1990). ''Studying Popular Music'', p.198. {{ISBN|0-335-15275-9}}.</ref> 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]].
==References==
{{reflist}}
{{Chord progressions}}
[[Category:Chord progressions]]