Chord substitutions

When playing a chord we have many different choices in how to outline that chord. An Am chord ( A C E ) could be Am6 ( A C E F#) or Am7 ( A C E G ) or Am9 ( A C E G B). When a chord in a progression is substituted for, or replaced by, a related but different chord it is called Chord Substitution. There are endless creative possibilities, one way of out lining the concept is looking at the basic II7 - V7 - I7 chord progression. In the key of G major that is Am7 - D7 - G7 / (A C E G) - (D F# A C) - ( G B D F). First we can see that in the Am7 chord there is a C major arpeggio, the relative major. In the D7 chord the F# diminished arpeggio F# A C made up of consecutive minor 3rd intervals. The G7 chord also has a diminished arpeggio within its notes. So long as there is a bass playing the root note of the chord other instruments may experiment with chord voicings and notes over those chords.

Substitution happens when the bass note in the root of the chord changes. Changing a chord at its most basic involves taking the notes of a know chord and changing it by one note in the bass. For example if we take the D7 chord D F# A C, and remove the A, D F# C, and then add an Ab in the bass we have the chord Ab D F# C, of Ab minor7#9. This could be thought of as a D7 chord with an Ab minor in the bass. Or better that in the Ab minor7#9 chord there is a D F# C arpeggio. So the V chord in the II - V - I progression could be substituted for a bII chord. II - bII - I or Am7 - Ab minor7#9 - G7.