Imagine the following progression:
C F D♭7 C
We typically just explain that third chord as a tritone substitution—and we stop there!—but this must be maddening for beginners, because we aren't actively explaining what it's a tritone substitution of.
Is there a common labeling system that explains what's being substituted? Instead of just labeling that chord, say, "TTS," are there any systems in place that would clarify this as a TTS of a G7?
I'm looking to use a system in my own teaching, and I'm curious if such a system already exists. In the absence of no other solutions, I'm currently leaning towards something like TTS(G7).
iiandIVor ofviandI. But we don't label these every time we see them, we just learn that they have a similar function. We certainly don't labeliiasvi/IVwithout some reason to think we've moved to a different tonal center. The analogy you raise perhaps argues for calling E♭7 in C major "sub(V)/ii" or "TTS/ii" because it is a substituted secondary dominant of D. – phoog Aug 17 '21 at 20:54