Chord substitution is the tool for changing the harmony of a song. You can add to or replace chords in a progression to create a different transition to the next chord or series of chords. You can group chord substitution into three broad categories. Chords can be preceded by a chord that is added to a given progression, replace a given chord with another chord, or alter the function of a given chord. There are a number of variations that can be created in each category but the end result will be changes in the songs harmony made by chords which are added, replaced or altered. I will briefly discuss the categories in the following paragraphs.
When a song uses a limited number of chords sometimes you can add interest by preceding the chords of the song with other chords. Some options for sources for adding these chords are the scale of the key and the scale position of the target chord, chords from the circle of 4ths or 5ths or chords added chromatically from below or above the root of the target chord. These concepts are introduced in the paragraphs that follow.
Using chords within the key of the song stepwise based on their numerical position in the scale. Using numbers, stepwise up the scale would be chords moving from 1 to 2 to 3 to 4 to 5 to 6 to 7 to 1. Stepwise down the scale would be chords moving from 1 to 7 to 6 to 5 to 4 to 3 to 2 to 1. If we apply this type of chord substitution to the key of C major the options would be C to D to E to F to G to A to B or the reverse order. This is not to suggest that you would use all of the chords of the scale in a single substitution. Application should always be based on the type of song and sound you want to produce. Use your understanding and your ear.
Circle of Fourths and Fifths
Chords moving from C to F or A to D or Bb to Eb are moving in fourths. If you reverse the order F to C or D to A or Eb to Bb the movement is in fifths. These movements are the most common sequences in modern music and provide a palate from which chord substitutions can be made.
You can approach most chords with a chord whose root is a half-step higher or lower than the target chord. The two most common substitutions are diminished chords and altered dominant chords. This type of substitution is commonly heard used for passing chords. Another application is the use of chromatic minor 9th chords to modify the sound of a section of a song or add movement in a section where a chord is played for more than one bar.
This concept involves using a different chord instead of the given chord. Once applied the replacement chord itself may lead to additional chords being replaced in order to complete the harmonic statement started with the first replacement chord.
Triad chords that have two or more notes in common can be used as substitute chords for one another. A triad has two notes in common with another chord. For example in a Cmajor chord, C-E-G, the 3rd and 5th of the chord are the 1st and 3rd of Eminor. More opportunities are available when you extend the chord. Using the extensions for a C chord 1-3-5-7-9-11-13 or C-E-G-B-D-F-A there are 6 chords within this one voicing. Within this extended chord you have an Eminor, Gmajor, Bdim, Dminor, Fmajor, and Aminor. Once again, this is not absolute. You still need to apply your understanding of chords, chord function and your ear to determine when to use this type of substitution.
You can use the key of the relative minor and major as sources for chord substitution. This somewhat relates to the common tone chore relationship. For example, Aminor is the relative minor of Cmajor. They share notes in common and Aminor is the 6th note of the Cmajor scale. The Aminor scale has the same notes as the Cmajor scale but produces different chords based on the root tones and scale intervals of the Aminor scale. A full discussion of scales requires a separate presentation. For this discussion just know that chords from the minor chord based on the sixth note of the major scale can be used as substitute chords.
Sometimes, subtle harmonic changes are desired. This can be accomplished without changing the root of the chord by adding extended chord tones. This is an easy way to add harmony to a song without drastically altering its quality. On the other hand, when being subtle is not the goal, changing chord quality or using chromatic alterations to chord tones can modernize the sound or take it in a different harmonic direction than originally written.
Change chord quality
You can substitute a chord by changing the quality of the chord. If you are playing a Cmajor chord, depending on the song and sound you want to create, you could change it to a Cminor, Cdim, C augment or even some type of 7th chord with the same root. You could play a C7, Cm7, or Cdim7. Each of these possibilities could lead to other substitutions based on the change made to the quality of the original chord.
Extend chord tones
While similar to changing chord quality, the distinction is the chord function is not changed. If the chord is major it remains major. If the chord is minor it remains minor. Extended chord tones are from the scale of the chord. It consists of adding the diatonic 9th, 11th, or 13th to chord changing its sound but its identity.
Chromatic Chord Changes
Chromatic changes go beyond extending the chord harmony. Chromatic changes alter the 5th , 9th , 11th, or 13th. These are the harmonies you most often hear in passing chords. However, these altered chords are used to harmonize melody notes and give songs a modern or jazzy sound.
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