Chord rewrite rules

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

Steedman (1984)[1] 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 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 Sequences by fourth, rather than fifth, include Jimi Hendrix's version of "Hey Joe" and Deep Purple's "Hush": These often result in Aeolian harmony and lack perfect cadences (V–I).

Middleton (1990)[2] 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."