I understand that a diminished 7th chord is defined by the intervals 1-b3-b5-bb7 and a half-diminished 7th chord is 1-b3-b5-b7, but how do you determine what intervals diminished (and half-diminished) 9th, 11th, and 13th chords have?

  • What style of music are you playing? I would guess Jazz, as that is the most common place you find people talking about extensions/alterations. Jan 5 '15 at 15:52
  • Actually I primarily play rock. I've just been looking at new fresh ways to spice things up and maybe sprinkle in a few "jazzy" chords. :) Plus I just find the theory very interesting regardless of whether it really applies to my playing style. Jan 5 '15 at 16:39
  • Related: music.stackexchange.com/q/74886/45266
    – user45266
    Jan 28 '19 at 16:53

The extensions are not touched when dealing with diminished chords unless noted. All extensions can be played as is with the only exception is the 13th chord which cannot coexist with a fully diminished 7th chord since the lowered 7th exists in the space of the 13th.

To make it easier, looking at how it is sometimes notated may help. The Jazz way to write half diminished chords with extinctions is to notate it as a minor chord of whatever extension with a flat 5. For example:

  • Cm9b5
  • Cm11b5
  • Cm13b5

A diminished chord itself has a nice property of being completely symmetrical In most cases you would not use an extension with a fully diminished chord because of the symmetric nature of the chord would be destroyed. You also cannot use a 13th because it is eharmonic with the diminished 7th. However, if you really wanted to you could notate it as a minor 6th chord of whatever extension with a flat 5. For example:

  • Cm6/9b5
  • Cm6(11)b5

These chords do not come up much in music so the names of these chords are not the best. Also both examples above use more of a "jazz approach" to naming chords. Personally, I would notate the chord you want with putting the diminished sign and then the extensions (C°9 or Cø9).

  • You can, however, use a b13. It doesn't clash with any of the notes in the chord and it's part of the whole-half dim scale, which is what the "colours" of a dim chord are derived from.
    – 02fentym
    Jan 5 '15 at 3:03
  • I actually have seen C°11 and Cø11 before written in that way: en.wikipedia.org/wiki/… The chords I am referring to are in the "11ths" section of the page. For a half diminished 11th it uses the intervals (1-b3-b5-b7-b9-11) for a full diminished it uses (1-b3-b5-bb7-b9-b11) I don't fully understand where these intervals come from though. Where do they get that it uses a "b9" instead of a regular 9? Jan 5 '15 at 5:13

The intervals are determined by the corresponding chord scales. However, since the chord scale is not unique, there are always several options. For a half-diminished chord a common chord scale is locrian. From locrian you get b9, 11, and b13 as tensions. You could also use locrian ♮2, which would give you 9, 11, and b13. A common chord scale for a diminished chord is the diminished (whole-half) scale, which gives you 9, 11, and b13. The tensions you mention in your comment (b9, b11) are very uncommon for a diminished chord.

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