I noticed something the other day about the relationship between jazz chords and scales. I first thought it was a shortcut for thinking about an appropriate scale over any chord on the fly, but I'm thinking it might be more fundamental than that.
To start with, here are the basic chord families found in most jazz. Beside each is the scale with all consonant notes. These aren't necessarily the best or most often-used scales, but it's generally accepted that every note in each of these scales sounds strong and consonant over its chord. It seems that there's a unique scale fitting this role for each chord family. This is probably an oversimplification, but bear with me for the purposes of my discovery.
- maj7 (1 3 5 7) => Lydian (1 2 3 #4 5 6 7)
- m7 (1 b3 5 b7) => Dorian (1 2 b3 4 5 6 b7)
- 7 (1 3 5 b7) => Lydian Dominant (1 2 3 #4 5 6 b7)
- m7b5 (1 b3 b5 b7) => Locrian #2 (1 2 b3 4 b5 b6 b7)
- dim7 (1 b3 b5 bb7) => Whole-Half (1 2 b3 4 b5 #5 6 7)
- aug7 (1 3 #5 7) => Lydian Augmented (1 2 3 #4 #5 6 7)
- m/maj7 (1 b3 5 7) => Melodic Minor (1 2 b3 4 5 6 7)
- 7b5 (1 3 b5 b7) => Whole Tone (1 2 3 b5 #5 b7)
- 7#5 (1 3 #5 b7) => Whole Tone (1 2 3 b5 #5 b7)
- 7b9 (1 3 5 b7 b9) => Half-Whole (1 b2 b3 3 #4 5 6 b7)
- 7#9 (1 3 5 b7 #9) => Super Locrian (1 b2 b3 3 b5 b6 b7)
Here's the pattern in constructing the scales from the chords. Start with the chord tones in the scale. Those will obviously be the most consonant. Now add the note one whole step above each chord tone. Whenever this results in consecutive minor 2nds in the scale, lower the new note by a half step. That's it. That works for every chord above, and it has held up with all sorts of complex extensions as well. See for yourself and let me know if you find any exceptions.
Shortly after discovering this pattern, I found a couple reasons why it works. The first relies on the concept that melody is just harmony in motion. When you play a melody, with or without accompaniment, you're implying different underlying chords or different voicings of the same chord. In a typical jazz setting, the bass player plays the lowest note of the chord, and the rhythm player plays more chord tones together, often omitting the root. The soloist emphasizes certain notes in the melody with their rhythmic placement. These notes, usually falling on the beat, at the end of a phrase, or held for longer than other notes, help define the current harmony along with the bass and rhythm parts. For example, bass plays root, piano plays 3 5 b7 9, and sax emphasizes a 6 in the melody. At that point, the current chord becomes a dominant 13 instead of just a dominant 9 from the rhythm section. It seems like the above scales work because any chords implied by using these scale tones as chordal extensions will remain in the jazz idiom. In other words, the above scale tones are the most common default extensions for their corresponding chords in typical jazz harmony.
The second reason I came up with is more compelling. It relies on the undesired dissonance of the minor 9 interval (octave plus a half step), played harmonically. There's a ton of tasteful dissonance used in jazz for creating tension or strange sounds, but the minor 9 interval seems to have an extra awkwardness about it when applied to harmony, I don't know why. Try it for yourself: play a maj7 chord with 7 in bass and root on top, or a 7#9 chord with #9 in bass and 3 on top, or a m9 chord with 9 in bass and b3 on top, etc. They all sound kind of awkward, especially compared to voicings without a minor 9.
Now, when creating the above scales by adding whole steps above the chord tones, all such minor 9 clashes with the rhythm section (usually an octave or more below the soloist) are avoided. Hurrah! This clears up a few other things for me as well, such as the 1 and 4 being weak points in a major scale over a maj7 chord. They create minor 9 intervals with the 7 and 3, respectively, in the rhythm section. Hurray! I've also found that a b2 over a maj7 chord sometimes sounds cool, albeit really weird and outside. And it's because it's a whole step above the 7. Hurroo!
While there are a number of exceptions to avoiding minor 9 intervals, and to my discovery in general, this still gives me a better grasp of consonance in melody. Besides, exceptions occur everywhere you look in music and music theory anyway.