Assume we are in the key of F. Then II = Gm7, V = C7 and of course I = Fmaj7.

If we want to include the 9th in the V chord then we have V = C9 = C E G Bb D. All of those note are in the scale of F major.

The question is why sometimes in Jazz people play a flat D, that is why they substitute C9 with Cb9?

Indeed it sounds nice since this b9 note goes nicely into the root but harmonically it is not within the notes of the scale.

  • Please ask independent questions separately. I have edited out the second question from this post, but please feel welcome to re-post it as a separate question. Jan 10, 2017 at 2:53

2 Answers 2


From your question, I guess you're coming from the theory point of view. It's being said more and more. Theory attempts to explain what's happening. NOT theory came first, so things ought to fit into its parameters.

If the latter was the case, we'd still be playing early madrigals; Bach, Beethoven, Schonberg, Debussy, et al, would never have emerged. Surely, the main reason we feel that something works is that it sounds good. It isn't always possible to pigeon-hole particular aspects, even though we seem to be programmed to need it done.

The main 'reasoning' here is that music needs (to us) to resolve, and moving each note in a chord as little as possible is a good way for this to happen. Almost like - you know the sounds you're expecting, but the chord before is so very close, but not quite there. A bit of a tease - dissonance, maybe.

C E G Bb Db. To resolve to F A C E involves C and E staying put, but the E can move one semitone to F. Bb coming down one semitone to A. Db dropping one semitone to C. G moves a whole tone - unless a jazzer would play the F as Fmaj9 ! Very little movement all told, but the blend of penultimate chord notes sounding good.

  • I have studied classical harmony so to me some of the "rules" of jazz are completely new. Now, in the specific case I indeed see, as I mention in my question, the Db resolves nicely to the C but I wanted to know if jazz musicians have a more "theoretical" way of thinking this. I see your explanation, and thank you very much. The resolutions I do get them, but constructing tensions like this is quite new to me.
    – Marion
    Jan 10, 2017 at 13:25
  • b9, #9, b5, #5, and combinations thereof, are some of the most used extension note changes (bad phrasing) in jazz, and all create tension that wasn't used much in classical.
    – Tim
    Jan 10, 2017 at 13:34
  • 1
    @Marion - Part of what makes this work is the chromatic motion in the line. As Tim mentioned, part of what makes all this work is minimal motion between voices, which creates a smooth transition from chord to chord. In this case, we end up with a chromatic line: D (5th of Gm7); Db (b9 of C7); C (5th of Fmaj7). If there were no b9, D would appear on the C7 as natural 9. The b9 adds increased tension and, therefore, increased resolution. It also "spices things up" a little, which Jazz loves. There are a lot of differences between Jazz and Classical, so don't hold on too hard to those "rules". Jan 10, 2017 at 14:25
  • 2
    It's a lot like Chromaticism in Classical theory, where you are looking to create a smooth linear motion as you borrow chords from the parallel key. If Classical had been using 9s more consistently and earlier on, I suspect that we'd see a lot of this there as well. Jan 10, 2017 at 14:28
  • @Basstickler - tritone substitution works in a similar way, too, as the target chord is preceded by a chord one semitone higher, the tts.
    – Tim
    Jan 10, 2017 at 15:43

It simply causes a little more tension and gives a bit more payoff for a resolution. Any V7 will want to desperately resolve to I. Adding a ♭9 adds more tension between the root of the V and the ♭9 and gives a bit more weight to the resolution.

On a typical resolution from C9 to FMaj7 you have:


On a typical resolution from C♭9 to FMaj7 you have:


The resolution is the exact same to the tonic, but the C♭9 has a slightly shorter distance to resolve back to the C being viewed as a stronger resolution.

As for what to play over it, Mixolydian ♭2 would be fine however with a chord like this you would want a little extra tension so playing just Mixolydian and chromatically going from D to C may yield a rather interesting result.

  • Also, since a 7b9 without the root is a diminished chord, all your diminished scales and arpeggios will work nicely, too. Jan 10, 2017 at 10:59
  • The dominant chord can resolve to the sub mediant chord as well.
    – Neil Meyer
    Jan 10, 2017 at 11:32
  • Yeah, I guess @Dom meant Fmaj7.
    – Marion
    Jan 10, 2017 at 13:21
  • Really don't like the naming. Cb9 used to catch me out in a number I used to play. Was it C (b9) or Cb (9). Context is all very well, but in the middle of sight reading something live, reasoning never came to me!
    – Tim
    Jan 10, 2017 at 13:24

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