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above is a picture of chord scales for primary dominant from the book I am reading.

Chord scale is just a scalar information of notes acceptable for making a chord.

The author says 5 and b13 cannot be used at the same time and does not explain why.

Can anyone explain why they are not used at the same time?

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  • In the context of chord voicing b13 is essentially #5 so the author most likely means that if the fifth is sharpened in a voicing it will clash with the perfect fifth
    – Jarek.D
    Commented May 20 at 11:52
  • with the same logic then how are b9 #9 both usable at the same time?
    – Sean
    Commented May 20 at 11:57
  • perfect fifth and sharpened fifth are semitone apart so very dissonant, whereas b9 and #9 are two semitones apart so less dissonant. So it is not the same situation. Alteration of chord is replacing the 5th or 9th with the sharpened or flattened tone and multiple alterations of the same note are accepted (though not that common)
    – Jarek.D
    Commented May 20 at 13:42
  • Big difference between b9 and #9 - a tone. Between b13 and P5, there's only a semitone. However, since they're an octave apart, seems no good reason to me. Playing a maj7 chord involves playing pitches 'a semitone ' apart. Best ask the author!
    – Tim
    Commented May 20 at 13:45
  • Some musicians "permit" (for lack of a better word) the sound of P5 and #5 together in dominant chords. So this may depend on specific person or style... Commented May 20 at 13:58

3 Answers 3

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I would be very wary of any theory advice that includes "cannot." Music theory, generally speaking, is studying what composers/musicians have done in the past to help you learn what you could do in the present. You can use them both, if you want. So, I would ask the teacher in what context he/she means you "cannot" use those notes. The answer might be...

  1. You cannot use those notes on the test at the end of the unit and expect to get that question right.
  2. You cannot expect to find those notes used together in any recordings in the specific subset of music you are studying.
  3. If you were to use them, they would sound too dissonant for this type of music.

For #1 above, this may be the reason. b13 is often used synonymously with #5, because raising the fifth here gives D# and lowering the 13th gives Eb. And usually when we alter an interval in the chord we stop using the unaltered one. So by treating it like a #5, we would avoid playing the regular 5th in the chord. This is a bit suspicious, though, because the 13th definitely assumes that the lower chord members are still present.

For #2 above, it is quite uncommon to have both a perfect fifth and a flat 13. Good luck finding one... But to say it never happens is not correct either. It's just rare, and dissonant enough that most musicians would hear it as a wrong note.

For #3 above, yes it will be very dissonant. It might sound like a wrong note, and might sound better to YOU if you omit the perfect fifth and let the b13 color the chord. But assuming you are composing, or improvising using the scale, it's up to YOU and not your theory teacher!

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If the author doesn't explain why, there isn't really a way to definitely know their reasoning.

Maybe the author does explain it, but in another part of the book.

One possible reason is found in the "avoid tone" concept in some jazz teaching, where scale tones to avoid are those that are one half step above proper chord tones. The flat thirteenth is one half step above the perfect fifth (disregarding octave placement) and is therefore an avoid tone.

You might ask about the flat ninth and why it is not an avoid tone when that ninth is one half step above the chord root? I believe the answer would be, because the flat ninth is considered a proper chord tone.

Another situation that could involve a problem with a flat thirteenth is something like a m6 chord but lowering that sixth a half step. (Understand the 6 could be expressed as a 13.) If you had base chords like Cm or Cm7 and then added the flat sixth or thirteenth, you would end up with tones C Eb G Bb Ab. That group of tones has the potential for the Ab, the flat thirteenth, sounding like the root, rather than C, in which case the proper naming of the chord would be Abmaj9. In this situation it isn't one of avoiding a tone, but one of misunderstanding the chord root.

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♭13 is ♯5. Of course you CAN use them together. The writer is warning you that, in their opinion, it won't be harmonically useful (or sound very nice).

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