How to Use Triads to Substitute for 4-note Chords    

Chord Substitution -  3-note(triad) substitution for 4-note chord

If you’ve been through the Prelude you are probably saying, “GREAT! I already know 3 chords.”  But what if I told you that you already know 12 chords? Would you be more excited? Just by changing the bass note on the left hand these three chords can substitute for other chords.
Let's take the C major triad (C  E  G). If you change the bass note on the left hand from C to A you now have the Am7 chord (A  C  E  G). Change the bass to F and you have the
F maj9 N/3 (no third) chord (F  C  E  G). Change the bass note to D and you now have the D11 N/5 (no fifth) chord (D  C  E  G). So now you know C, Am7, Fmaj9 and D11.
And you can do the same thing with the F major triad (F  A  C). Change the bass note from F to D and you have the Dm7 chord (D  F  A  C ). Change the bass to Bb and you have the Bbmaj9 N/3 (no third) chord (Bb  F  A  C). Change the bass to G and you now have the G11 N/5 (no fifth) chord (G  F  A  C). So now you know F, Dm7, Bbmaj9 and G11.
And you can do the same thing with the G major triad (G  B  D). Change the bass note from to G to E and you have the Em7 chord (E  G  B  D). Change the bass to C and you have the Cmaj9 N/3 (no third) chord (C  G  B  D). Change the bass note to A and you have the
A11 N/5 (no fifth) chord (A  G   B  D).  So now you know G, Em7, Cmaj9 and A11.

The rule for 3-note(triad) substitution for 4-note chord is as follows:

6th chord use the triad a 3rd down

7th chord use the triad a 3rd up

9th chord use the triad a 5th up

11th chord use the triad a b7th up

13th chord use the triad a 9th up.

Examples of some common chords.

6th chord use the triad a 3rd down

C6           =  Am  

Cm6        =  Adim.

7th chord use the triad a 3rd up

CMaj7      =  Em

C7            =  Edim.

Cm7         =  Eb

Cdim7      =  Ebdim., Gbdim.,  Adim.        (equal interval chords)

Cm7-5     =  Ebm

9th chord use the triad a 5th up

CMaj9      =  G

C9           =  Gm

Cm9        =  Gm

C7-9       =  Gdim, Bbdim., Dbdim.

11th chord use the triad a b7th up

C11        =  Bb                                  (C11 is sometimes written Bb/C)

C11b9    =  Bbm

C11#5    =  Ddim

13th chord use the triad a 9th up

C13        =  Dm  

C13b9    = Db+ 

 Note: There are other 3-note substitutions for some of these chords but they are not named triads.

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