I am confused by some of the chord substitutions in "serious" jazz. I get tritone substitution OK. What confuses me is why you're allowed to do things that have not only no notes in common with the original chord, but would in fact produce horrible dissonance when played with the original chord. Because if you the bass player are trying to play a Gm9 (G Bb D F A) and the piano player decides to play something like Do (DFAbB) instead, you have two potential half-step clashes. But I see this in transcriptions all the time. Not that particular example which is entirely made up, but stuff like that. And I have bass transcriptions by themselves where the bass player will just play a B on the downbeat of a measure of a Bb something. Like, often. Not as a b9, but as some mysterious other thing. Like maybe it's a Bo instead of a Bb? Or whatever. Don't take my examples as my question. My question is what in general are the rules for chord substitution beyond tritone substitution on the dominant.