The Complete Idiot''s Guide to Music Theory

Part 5:
Embellishing
206
Examples of compound chords.
When you play two chords together like this, you have what’s called a 
compound
chord.
You use compound chords to create extremely complex harmonies—
those that might otherwise be too complex to note using traditional extensions.
One Good Chord Can Replace Another
When you’re faced with a boring chord progression, you may have no alterna-
tive but to substitute the chords as written with something a little less boring.
The concept of 
chord substitution
is one that is common in jazz (those jazz musi-
cians get bored easily!) and other modern music.
Chord substitution is a simple concept. You pull a chord out of the song, and
replace it with another chord. The substitute chord should have a few things in
common with the chord it replaces, not the least of which is its place in the
song’s underlying harmonic structure. So if you replace a dominant (V) chord,
you want to use a chord that also leads back to the tonic (I). If you replace a
major chord, you want to replace it with another major chord or a chord that
uses some of the same notes as the original chord.
The key thing is to substitute an ordinary chord or progression with one that
serves the same function, but in a more interesting manner.
Diatonic Substitution
The easiest form of chord substitution replaces a chord with a related chord
either a third above or a third below the original. This way you keep two of the
three notes of the original chord, which provides a strong harmonic basis for
the new chord.
This type of substitution is called 
diatonic substitution,
because you’re not alter-
ing any of the notes of the underlying scale; you’re just using different notes
from within the scale for the new chord.
For example, the I chord in any scale can be replaced by the vi chord (the chord a
third below) or the iii chord (the chord a third above). In the key of C, this
means replacing the C Major chord (C-E-G) with either A minor (A-C-E) or E
minor (E-G-B). Both chords share two notes in common with the C chord, so
the replacement isn’t too jarring.

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Chapter 16:
Chord Substitutions and Turnarounds
207
Replacing the I chord (C Major) with the vi (A minor) and the iii (E minor)—lots of notes in
common.
You can replace extended chords in the same manner, and actually end up with
more notes in common. For example, you can replace CM7 with either Am7 or
Em7, both of which have three notes in common with the original chord.
Major Chord Substitutions
Diatonic substitution is the theory; you’d probably rather know some hard-and-
fast rules you can use for real-world chord substitution. Don’t worry; they exist,
based partially on diatonic substitution theory.
The following table presents four different substitutions you can make for a
standard major chord. Remember that the root of the substitute chord must
stay within the underlying scale, even if some of the chord notes occasionally
wander about a bit.
Major Chord Substitutions