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).