Contents:
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General Chord Substitution
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Extending Chords
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Embellishing Chords
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Combining / Conclusion
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Common Tone Substitution
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Barrowing From Parallel Keys
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Tritone Substitution
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Random Chord Substitution / Conclusion
General Chord Substitution
Replacing a chord in a progression with another similiar chord is known as chord substitution. Chord substitution a great way to spice up cover songs to make it sound a little different than the original and also to make your own songs more interesting. The chord that replaces the original chord generally has some relation to the original.
Extending Chords
Extension chord substitution involves extending the triads by
adding extra notes to the triad, such as the 7th, 9th, 13th, etc. scale degrees to the chord.
NOTE: There are only 7 notes in the scale, so anything above 7 is a repeat of one of the scale degrees from 1 to 7. For example, the 9th is the same as the 2nd, the 13th is the same as the 4th, etc...
This process is pretty self explanatory, but what happens when you get up to the larger chords like 13th chords? It consists of 7 notes total, (1st, 3rd, 5th, 7th, 9th, 11th, and 13th) and there are only 6 strings on the guitar. This is where you start eliminating certain notes. In most cases, the 5th is the first to go. After that I think it becomes a matter of preference and which voicing sounds the best for the song.
"Add" chords are another way to extend a triad. An add chord simply adds the specific scale degree to the chord. For example a Cadd9 adds the 9th scale degree and the notes are C E G D.
If there is no "Add" in the chord, then you must
include all of the notes created by stacking thirds onto the triad up to that specific scale degree. In other words, the Cadd9 chord differs from the C9 chord because the "add" tells you to only add that one specific scale degree (the 9th) to the chord, whereas the C9 chord also adds the 7th scale degree in addition to the 9th. Chords that don't have "add" are built by stacking thirds to get to the specified note.
13th chords are as high as you can go by stacking thirds. The 15th is a repeat of "1", so you would be starting all over again if you went any farther. To get to the 13th, you would have to stack thirds all the way up to the 13th scale degree:
1 3 5 7 9 11 13
For example a C13 chord is C E G bB D F A.
NOTE: The exception to the above rule is the 6th chord, because these cannot be built by stacking thirds. Therefore C6 only adds the 6th scale degree and nothing else (C E G A).
Embellishing Chords
By embellishing chords, you
alter existing notes in the chord.
Suspended chords (abriviated "sus") change the 3rd in the chord to either a 2nd or a 4th. In other words, for sus the 3rd is not played in the chord. Sus chords are therefore neither major or minor, since the 3rd is what determinines if the chord is major or minor. You can see a comparison between these various C chords.
C major: C E G (E is a major 3rd)
C minor: C Eb G (Eb is a minor 3rd)
Csus2: C D G (there is no 3rd)
Csus4: C F G (there is no 3rd)
Augmented chords (abbrevieted "aug" or "+") sharps the 5th scale degree. This is a very short and simple explanation, because there's really not much to explain. Here is an example of the augmented chord:
C major: C E G
C+: C E G#
Combining / Conclusion
We have just covered two ways to replace basic triads with more complex chords. As you may have guessed, you can also combine both the extension and the embellishment methods to create all kinds of chords like C7sus4, or F#13b5b9, etc.... The possibilities are pretty much endless.