Wonderful site! I'm really enjoying this lesson, but do have a question (doubtless, something obvious I'm missing) about substitution number 8. I understand that the C# dim. functions as an A7b9, the sixth chord of the progression. Further, it's I clear that the D#dim can be substituted for a B7b9 on the same principle. What I'm uncertain about is the function of the B7 chord in the progression. Specifically, the C# dim is functioning as the 6th chord, but what is the function of the B7b9 (for which the D# dim is being substituted) in the progression? As a five of five (the E shows up as a substitute for the I chord, C maj) I can sort of see the connection of the B chord, there but don't really understand how the B fits in the mix.
Thanks so much, and kindest regards, Steve K.