C Dorian is useful when playing over Cm7-F7, a chord progression that has an A note. What's the correlation?

Question

http://taooftunings.com/book/major-scale-modes-and-chords/

I read from the above site about major-scale-modes-and-chords and it's talking about the Dorian scale.

Dorian C – D – Eb – F – G – A – Bb – C

1 – 2 – b3 – 4 – 5 – 6 – b7 – 1

"C Dorian is useful when playing over Cm7-F7, a chord progression
that has an A note."

Cm7 --> C Eb G Bb

F7 --> F A C Eb

I understand that F7 has the A note. But why is C Dorian "useful" when playing over Cm7-F7, what does it have to do with F7 having an A? Or can I also say it's good for playing Cm7-XXX where XXX is anything that is derived from Bb scale major that has an A?

Answer 1

C Dorian uses the notes from its parent scale/key of B♭. Cm7 is the chord based off the second degree of B♭, and F(7) is the dominant. So using C Dorian instead of the possibility of using C minor is a better bet. Reason is that C minor contains A♭ instead of A, which is found in C Dorian. Put another way, in a ii-V-I sequence in B♭, there's Cm-F-B♭. All that's missing in your sequence is the final B♭, making the piece sound as if it's in Cm, but actually C Dorian is a better fit.

Having said all that - the melodic minor version of Cm does actually possess an A rather than A♭ note!

Answer 2

As Tim suggests, in a Cm-F7 progression, it is desirable to use Dorian minor and F Mixolydian because they both derive from Bb maj. Playing C Aeolian minor, by contrast, would not fit in the Bb maj parent scale.

More generally, the reason this will sound good is because it enhances melodic continuity between the two chords. When playing C Dorian and F Mixolydian, the A is a shared tone. C Aeolian, by contrast, would have one fewer shared tone due to the presence of the Ab.

So the general principle being relied on here is: it will usually sound good to choose the mode that enhances continuity. So you are correct that it will sound good to choose C Dorian in any scenario involving Cmin-XXXX where XXXX is derived from the Bb maj scale. But we could extend the rule more broadly than that.

For example, let's imagine a Cmin-A9 progression. The A9 chord is a dominant 7th chord with a natural 9th. C Dorian would sound good because it has an A, which would be shared with A Mixolydian. But if the progression instead were Cmin-Ab7, then it might be better to play C Aeolian, because then the Ab will be shared with whatever scale is played over Ab7 (perhaps Ab Lydian Dominant or Ab Mixolydian).

So as a general rule, one approach for choosing a mode is to maximize the shared tones with the next chord.