I see a LOT of pages and books showing which enclosures work for certain chords and target notes, but I haven't seen many explain why a given enclosure works over some other arbitrary set of approach notes. I get the sense not all chromaticism is equal, so is it just a matter of taste and avoiding the avoid tones in a chord? Or is there a better way than memorizing dozens of enclosures?
I found this clinic video very useful.
The term "enclosure" isn't in the slides, but the embellishments discussed are enclosures and the demo examples are jazz, When the Saints Go Marching In and Cole Porter What Is This Thing Called Love.
The difference between this clinic and the typical jazz web tutorial is the clinic clearly puts things in terms of diatonic scales. That helps a lot with understanding upper/lower, half-step/whole-step enclosure options. It seems the upper tone is the one that gets special attention. If the the upper half step is diatonic, then it can be the one upper enclosure tone. But if the upper half step is chromatic, it gets preceded by the diatonic tone above it. By comparison the lower half step can be either chromatic or diatonic.
That nutshell description is about the contents of the clinic video and slides. Someone else will say you can do anything you want, and you can. But your question is asking for some reasoning behind how enclosures are handled. I think the clinic will provide a nice grounding.
Here are some general principles for enclosures:
- they approach the target both from below and from above
- they usually include 1-2 approach notes below and 1-2 approach notes above the target
- they can "change directions" (ascending/descending) as many times as you want*
- the approach notes can be constructed using chromatic/half steps or diatonic/scale steps
- there is a slight bias for chromaticism when approaching from below and diatonicism when approaching from above
So you are exactly right that all chromaticism is not equal! I call these biases instead of rules because you'll find tons of counterexamples. But when we listen broadly and look at how frequently these enclosures are used in bebop, the slight bias is heard.
Let's apply this slight bias for chromaticism/diatonicism to increasingly complex enclosures. In all cases, we'll use each note of the CMaj scale as our target.
*Note: I'm calling the enclosure "ascending" if we play the higher approach notes first, and then play the lower approach notes second.
You list many questions.
The underlying theory? I don't think there are any. The bebop musicians just started making small phrase fragments that sounded cool, created harmonic tension ("horizontal" playing) and was easy to do in fast tempos.
The enclosures, or "hinges" they used to call it, will end up on most frequently the chord tones, frequently the 3rd (possibly due to the blue notes around it).
Now, the notes used in the hinge can be chromatic and scale based, and I think the most common practice is following the mode scales, modified by the scales used on altered dominant chords.
In my opinion, since there is no theory behind it (also my opinion), I'd recommend not memorizing it. It is much easier to start using it by "prehearing". Singing in unison with your instrument can be very worthwhile. Start experimenting the hinges around the chord notes and see what you like, and what sounds cool. Try ending the hinge on different parts of the bar, and try ending it on up beats as well as down beats.
The personal taste of a soloist is a part of the magic of jazz music, I'd say, and the only way to use it is practicing, and letting your ear decide.