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I believe that 9ths are not commonly added to half diminished chords. If we wanted to write all the diatonic 7th chords of the E♭ Major scale, with the 9th added, we would have for example on the VI degree the notes C, E♭, G, B♭, D (the 9th), making it a C-9 chord, but the VII degree would be D, F, A♭, C (with no 9th added), becoming D-7♭5.

I understand that any notes are permitted in music, but, why is this common practice? Is this correct?

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  • Hi David -- I mentioned that I thought it was common practice not adding the 9th because I don't see it very much, but if musicians do play it in a half-diminished chord context, why aren't they written? It has to do with some dissonance avoidance rule like a minor 2nd or tritone? Is there something «slippery» about this 9th?
    – Marco A. Ferra
    Commented Mar 24, 2018 at 15:30
  • It seems like I have heard m9♭5 chords in the wild, and I think that I have occasionally played them, but maybe you are right that they aren't that common. I checked Ted Greene's Chord Chemistry, which does not list this chord and says: "the only altered minor chord that is widely used is the m7♭5 type."
    – user39614
    Commented Mar 24, 2018 at 15:39
  • Thanks for the book reference David, I'll check it to learn a bit more about chords. In the meanwhile, if anybody has some explanation why this 9th isn't added, it would be very appreciated.
    – Marco A. Ferra
    Commented Mar 24, 2018 at 15:44
  • If you are looking at diatonic 7th chords, then the ninth chord built on the 7th degree of E♭ would be Dm7(♭5♭9), which sounds pretty dissonant. The ♭9 is considered an avoid note of the Locrian mode. But Dm9♭5 can be constructed from the 6th degree of F melodic minor (Locrian #2), which has no avoid notes.
    – user39614
    Commented Mar 24, 2018 at 16:03
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    The obvious counterexample is a dominant 7♭9 chord. These are used all the time and sound great.
    – user39614
    Commented Mar 24, 2018 at 18:29

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As mentioned in the comments, what you would get is a b9, which is an avoid note for minor chords, i.e., chords with a minor third (regardless of the 5).

Note that you get the same problem with the III chord (Gm7 in the key of Eb major). Its diatonic ninth would be an Ab, which is the b9, and again it is not used, because also here it is considered an avoid note. So it is not only half-diminished chords that usually will not be played with a b9, but also minor seventh (m7) chords. Of course, in both cases you can decide to use a non-diatonic (major) 9th, but that's a different story.

The only chord type where a b9 is used on a regular basis is the dominant seventh chord, especially if it resolves down a perfect fifth, because there the added tension is desirable.

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    Excellent -- I did noticed the same dissonance when adding the 9th to the Gm7 chord in the key of E7 major, but in the half-diminished chord seemed even more harsh, perhaps just my ear. Thank you, Matt, and all that commented, it's clearer now.
    – Marco A. Ferra
    Commented Mar 26, 2018 at 8:00
  • I also discovered in the book "Modern Harmony Step by Step" by Ricky Schneider that indeed the 9th are avoided notes in III- and VII- degrees. Screenshot: i.imgur.com/GEOnKVh.png -- the book mentions in the previous page that a tension is available when it forms an interval of 9 with one of the notes of the chord and not available when it forms one of b9. Could this rule come from classical music?
    – Marco A. Ferra
    Commented Mar 27, 2018 at 19:53
  • @MarcoA.Ferra: I don't know that book, but in practice that rule is not followed. As I've mentioned in my answer and elsewhere, the b9 is definitely available and used a lot with dominant seventh chords. There are also other examples such as major add 4 chords where there's a half step (or a b9) between a chord tone and the tension.
    – Matt L.
    Commented Mar 27, 2018 at 20:19

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