You can’t always write a chord ugly enough to say what you want to say, so sometimes you have to rely on the giraffe filled with whipped cream.
–Frank Zappa
In the video below guitar teacher Robin Nolan fields a question about what scales one should learn in order to play gypsy jazz. Nolan’s answer is “major and minor scales”, by which he means don’t bother learning scales beyond the bare minimum that you probably already know. He then goes on to discuss how it doesn’t make sense to think at the level of a scale. What you have to do is to get to know a scale’s individual tones. So the sixth note of the major scale has that “sixth” sound. The ninth note has that “ninth” sound. And so forth. Nolan makes a valiant effort to explain things better than this, but ultimately just settles for playing various notes and saying, “See?”
Put into words, Nolan’s instruction sounds vacuous to the point of tautology, but the thing is, he’s right. The mindset he describes here is pretty much what I have when I play lead guitar, and I’m sure plenty of players a lot better than me. What’s striking is how ineffable it all is. There are sounds that evoke certain musical moods, and we assign those sounds numbers or solfège syllables or what have you, but those are just mnemonics for where to put your fingers. The sixth tone is not twice as evocative as a major third along some axis of expressiveness. Words fail us quickly. The only way to know is to play.