The chords are interchangeable because the tritone interval pitches are identical in each (G is a tritone above Db and G is a tritone below Db).Here is a list of the possible Tri-tone substitutions:A7 <-> Eb7B7 <-> F7C7 <-> Gb7D7 <-> Ab7E7 <-> Bb7G7 <-> Db7There are many other types of substitution (huge topic).Rodney