The Music Stem / The Musician's Library / Composer's Handbook / Tritone Substitution
Composer's Handbook
A tritone substitution replaces a dominant seventh with the dominant a tritone away.
Replace G7 with D♭7 before Cmaj7 and compare the bass.
G7 (G–B–D–F) and D♭7 (D♭–F–A♭–C♭) share F and B/C♭. Bass D–D♭–C replaces D–G–C.