Tritone substitution · Music theory
Why D♭7 can take the place of G7, how it is used, how it combines with related IIs and what it has to do with the classical augmented sixth.
The same tritone
What makes G7 a dominant is the tritone between B and F. And another dominant seventh chord has those same two notes: D♭7, with F as 3rd and C♭, sounding like B, as 7th. The two dominants are a tritone apart and share theirs, with the roles swapped.
Since they share the essentials, one can replace the other: they resolve to the same tonic. This chord is called the tritone substitute, written subV⁷.