photo by Ed Vill
There are some subjects you learn in music school that fall into the category of Things I'll Never Use in Real Life. Included are things like memorizing all of the modes of major, harmonic minor, ascending melodic minor, and harmonic major in order from brightest to darkest....yes I'm serious. That's 28 different modes. What's an Ionian sharp 1 anyway? (if memory serves, it's the mode built off the 7th degree of the melodic minor scale)
In order to pass these tests you must master the art of mental regurtitation. Attempting to hold on to such arcane factoids for more than about 90 minutes(the time it takes to cram before the test plus the time to puke out the answers) can be hazardous to your health.
Then there are subjects that are useful, but you never expect to have a conversation about them outside of the classroom. Well it must be International Esoteric Jazz Theory Day, because I had a guitarist friend of mine say, "Hey man, I'd like to pick your brain some time about slash chords."
I assumed his question was the only one I frequently hear asked; "what is the difference between horizontal and diagonal slash chords?" It wasn't, but here's the difference just in case you're wondering.
I agreed, because the A natural isn't present in a D lydian augmented scale. Let me say this before it gets too far into jazz nerdom. You can play whatever you want to against any chord. There's no right and wrong per se. When people ask, "What can I play over X, Y, or Z?" they are really asking, "What scale(usually a 7 note scale) will be most consonant with X,Y, or Z?."
Although it's still subjective, we can better answer the latter question. I think there may be 3 criteria for finding the most consonant scale.
1. How many notes are in the scale?
2. How many notes in the chord are in the scale?
3. How conjunct are the scale-tones between chord-tones?
I guess a common-sense fourth criteria would be whether or not it sounds "good." If we address the first 3 criteria, there will normally be some consensus on which scale is most consonant against any given chord.
Answering criteria #1, usually scales have more notes in them than the chord does. Otherwise, they would just be arpeggios. So, with the example given above, we start with a chord that has 5 tones. You could generate a 5-note or 6-note scale, but most musicians are interested in making a 7-note scale. You could even make a 12-note scale, i.e. the chromatic scale.
Often, music students sitting in a jazz theory class try to generate scales for a given chord by picking from those scales already memorized. Its as if they are pulling sockets out of their tool kit, trying to find the one that will fit just right. Sometimes it's better to start from scratch. Criteria #2 asks how many chord tones are included in the scale. If you hunt for a perfect scale, you might get lucky and find the one that fits, or you can simply start with the arpeggio and connect the dots, which brings us to criteria #3.
Essentially, any chord consisting of 4 or more tones will probably not require more than one passing tone between each chord tone. A triad is another story. Most of the time, the most consonant scale is produced by avoiding large intervals. Take the chord C7 for example:
The first example is the most common solution for C7. It consists entirely of major and minor 2nds. The second and third scale are both viable options, but will sound more dissonant to most people. The reason is because of the large minor 3rd intervals, Db to E or C to D#.
Then, if we connect the dots as evenly as possible(criteria #3) we generate the following:
Some call this scale D harmonic major. Notice that it contains the minor 3rd interval between A#(Bb) and C# which gives it an exotic and more dissonant sound. To avoid this interval would require that we either add additional notes to the scale(criteria #1) or not include all of the chord tones(criteria #2). From this example, we can conclude that the chord itself contains intrinsic qualities that influence the sound of the corresponding scale.
Another solution for this chord is often given. The D augmented scale is sometimes used. It's a 6-note scale(criteria #1). Unlike the D harmonic major scale, the D augmented scale contains no major 2nds. Instead an extreme alternation between minor 2nds and minor 3rds creates a symmetric scale.
The symmetry seems to be the attractive quality about this scale. Although I was initially fascinated by the augmented scale, I soon found it difficult to use in a melodic way. If you're not hearing it, don't play it.
Lastly, I'd like to point out that this notion of chord/scale relationships is a relatively new one. In jazz, it is generally recognized that musicians improvised using chord arpeggios and blues riffs and began to focus on scales later on in the history of jazz. Some point to George Russel's Lydian-Chromatic Concept as the turning point.
Many times, horn players will come up to me and say something like, "you piano players always play different lines. It's because you have the chords in front of you." This is to say that because we play the chords so often, we have the chords in mind more than a scale. Maybe so. Although it's useful to know how to generate a scale to play with a given chord, I don't think it's always as important as people think. And it's certainly not the end of the road. What you do with the building blocks is more important than if you have the exact correct sized blocks.