I recently became aware of the teaching videos of legendary pianist, Barry Harris. What an incredible educator. In the video below he talks with his students about how chords function. He asks, in how many major keys will you find a Cm7, CMaj7, C7, and Cø7?
The answer is important because it demonstrates that you can’t approach every Cm7 the same way. Barry explains that playing the note D over Cm7, for example, is incorrect in the key of Ab Major. There is no D in Ab Major so it doesn’t work. In Bb Major, however, it works beautifully.
Thus, if you know how the Cm7 is functioning in the song you can easily know which major scale to improvise with. If you study jazz this might seem obvious, or maybe even an over simplification. However, I think it’s a great way to conceptualize it. This concept becomes even more powerful when you realize that any chord can be thought of as either I, IV, or V. This is because related chord functions share notes.
For example, iii7 is related to I∆7:
iii7 (Cm7 — C, Eb, G, Bb) share 3 notes with I∆7 (Ab∆7 — Ab, C, Eb, G). Therefore iii7 and I∆7 can be used as substitutions of one another.
So if can get good at recognizing chord functions and their I, IV, V relationships you’ve completely simplified the process of improvisation. It’s definitely worth thinking about. Here’s an overview to get you started.
How many major keys is a Cm7? — Three
- the ii7 of Bb Major — ii can sub for IV (Eb)
- the iii7 of Ab Major —iii can sub for I (Ab)
- the vi7 of Eb Major — vi can sub for I (Eb)
How many major keys is a CMaj7? — Two
- the I∆7 of C Major
- the IV of G Major
How many major keys is a C7? — One
- the V7 of F Major
How many major keys is a Cm7(b5)? — One
- the viiø7 of Db Major — can sub for V (Ab7)
- (In the video he mentions that Thelonious Monk thought of this chord as a min iv chord with the 6th in the bass. Cm7(b5) = Ebm/C)