Steve's Spinal Column #17

To Extend Or Not To Extend, That Is The Question
by Tex Apport

The Full Sticktet

A common change or substitution for a chord would be to add an extension. The basic triad is 1 - 3 - 5 of some sort. We can of course add the 7th. But for even more flavor a ninth, and even the eleventh or thirteenth can be thrown into the harmonic mix. Let's explore the ninth as an addition to a chord.

C minor = C  Eb  G ( a very basic triad)
          1  b3  5
 
C minor 7 = C Eb G Bb ( a bit more sophisticated)
            1 b3  5 b7
 
C minor 9 = C Eb G Bb D (now we're talkin')

We know that diatonically, a C minor chord exists as ii, iii, or vi, in a Major key.

 
Therefore C min7 can be ii in Bb Major
                        iii in Ab Major
                        vi in Eb Major

Here are these 3 major scales:

Bb Major = Bb C D Eb F G A Bb

Ab Major = Ab Bb C Db Eb F G A

Eb Major = Eb F G Ab Bb C D Eb

We can see that Cmin7 (C Eb G Bb) exists in each of these keys. But if we add the 9th to the chord, the corresponding note, D (9th) only exists in the keys of Bb and Eb. In the key of Ab the D natural does not exist.

So we can summarize that a minor chord can be extended with a 9th, only if the chord functions as a ii or vi in a Major key. if it is a iii chord the natural 9th will conflict and hence not be harmonically correct. For the more sophisticated, we can also say the minor 9 can exist as the root of the Dorian Mode (ii) or the root of the Aeolian Mode (vi). We can't use the ninth in the root of a Phrygian Mode (iii)

Let's try the same exploration with a minor 6th chord.

Cminor 6 = C Eb G A
           1 b3 5 6

Now the 6th (A) does not exist in the keys of Ab or Eb Major. It will only fit into the Bb Major scale. Therefore the C minor 6th can only exist in Bb Major.

Conclusion:

Diatonically, a minor 6th chord can only exist as a ii in a Major key. Or as a root in the Dorian Mode (ii)

You can try this on your own with all the extensions, 7, 9 ,11, 13 (6) Of course you should try them on all chord qualities as well, Major, minor and dominant.

Here's a bonus concept:

The Cminor 6 = C Eb G A
               1 b3 5 6

If we rearrange the letters we can get  A  C  Eb  G. This is called A half diminished 7
                                       (1  b3 b5  b7)

F9 = F A C Eb G.... As you can see without the F, we get the same 4 notes as the previous two chords.

All three chords, Cmin6, A half-dim.7 and F9 exist in the key of Bb Major.
Cmin 6 is ii in Bb
A half-dim7= vii in Bb
F9 = V in Bb

RULE:
In any Major key: iim6 = vii half-dim7 = V9.

You can use this valuable information for substitutions.

i.e.
l C min7 l F7 l Bb Maj7 l Bb Maj7 l  ( a typical progression)
  ii       V    I         I

We can fancy up (extend) F7 by making it F9. Well, if we don't know many F 9th inversions we can use A half-dim7 chords or C min 6 chords instead and get the same results. Needless to say, you still need a good chord vocabulary, but now you have more options.