I understand that a little...Many times I sit in a jam, they call out G major and then the basic changes, 1, 4, 5 etc...or whatever the case, and we take off...I think we were thinking you were just guessing the changes...which could go really bad really fast...Yes, one example, the armenian jazz pianist Harry Tavitian invited me to play with him, I never played with him before, I didn't know his repertoire, and my first concert with him it lasted three hours. I didn't do any rehearsal before the concert and after he hired me for the whole tour.