It appears to me that you are the only one in this thread so far who interpreted my question at a deeper level, not superficially. Majority will view my problem through the prism of "fun" concept: If you like a piece just play it without trying to understand why it was composed using specific rules and limitations.
I'm mostly interested in improvisation: both Classical and Jazz. With Jazz it's much more easy to approach this subject. In Classical improvisation which now had been brought to a new level of understanding with the recent research efforts it's much easier to make a mistake and make the whole thing sounding incorrect, out of style. Perhaps the majority of potential listeners won't even notice when a 7th interval will be introduced without preparation as a larger than a 2nd interval jump. Some will probably even think: "That's a Cool sound!" while in fact it would be probably more correctly to call it "a harsh and vulgar sound".
The 7th was prepared in the CPP period (though at its earlier stages) for a very specific reason: if it's harsh and rough introduce it with elegance and subtlety.
Once the music went from the noble society into the masses it began to satisfy appetites of people who wanted more surface level fun. Surely the 7th introduced without preparation is much more fun! That's just one of the possible examples that demonstrate how music gradually lost it's grace and elegance.
Take Rag Time for example: what is it? If you approach it without thinking you may think: "What a nice syncopated style!". However if you think deeper you'll realize that it's a very arrogant and vulgar music and at the same time it's so simple and empty that once the syncopation is removed it turns into the worse childish tune that can only be imagined.
(I like Rag Time by the way! It's so cool!)
I said all that only to reiterate: it's very hard to cope with the vulgarisms and liberties of Jazz (and other popular styles for that matter) once you understand the subtleties and depth of the Classical approach.
I'm not against Jazz. In fact I like it's "vulgar and rough" approach (compared to the CPP music) but that's another person in me who likes it that way.
So, you see: these are two different people who need to be reconciled. How to do that?
Lastly I want to clarify my position regarding Jazz as a whole and especially its part that deals with a solo harmonic improvisation. It's an art form and it's not a silly superficial music like Rag Time (which I like for what it's worth). There is not much difference on the voice leading level or general harmonic approach between Jazz and Classical though they certainly differ on a structural level.
What I find difficult when switching between the two is abandoning the subtle rules that were idiomatic to the CPP music. It feels like a contradiction on a deeper level of understanding.
...The treatises are best observed as composers passing on what worked for them to the next generation of composers; the avoidance of the tri-tone in choral music being one of these examples (the commonly cited thing about the Catholic Church banning the tri-tone is false).
The important thing to know is that music did not stem from treatises. Rules did not make music, music made rules.
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