THEORY 42: Chord Substitutions 6
Chord Substitution is a space in which I enjoy to return to. The goal is to find that sublime chord that can so magically transform an evolving progression.
It occurs when an existing chord within a progression is replaced by one or more of the following diatonic or non-diatonic variations:
* An Extension - where additional intervals are stacked upon a triad
e.g. Am (spelled A C E) becoming Am7 (A C E G) by stacking the b7th (G).
* An Inversion or Slash chord - where the notes of a chord are
rearranged so that the root changes to another of its intervals, e.g. Em7 (E G B D) becoming Em7/G by changing its interval structure to G B D E.
* A Polychord - where a chord is perceived as a composite of two or
more separate chords, e.g. Am13 (A C E G F#) could be viewed as an Am7 (A C E G) combined with an F#mb5 (F# A C), perhaps even a C add #11 (C E G F#). In this case one of the chords within replaces the outer.
* An alternative Voicing - where the position of the chord is changed
to produce a new intervallic structure, e.g. move an open Em (E B E G B E low to high) to the 7th position (E G B E G). The name remains the same but the sound is transformed.
* A different Tonality e.g. Bm7 becomes B7 by sharpening D to D#. This approach can introduce non-diatonic intervals but if the notes emerge from the Harmonic or Melodic Minor scales then technically the notes are still diatonic.
* A Synonym - chords can be given more than one name, e.g. G6 (G B D E) has the same intervals as an inverted Em7 (E G B D). Using this can encourage the substitution. In this example, it only matters if the bass player changes the root from G to E.
* A Passing chord generally shares some common notes with the key e.g. G#7 (G# D# F# C) has two common notes with the key of Em. They can substitute an existing chord with common notes or fit in and amongst the others e.g. in the progression Em G Am D7 C maj7 G#7 Am.
In order to illustrate these concepts I have generated a basic I III IV II V diatonic progression G Bm C Am D in the key of G. Also bear in mind its relative key Em because even though it has the same notes and chords, this is where the Harmonic and Melodic Minor variations emerge from and they are a vital source of goodies.