I have learned that the dominant seventh chord can be written with a flatted fifth, for example
G-B-Db-F in C major. Recently, I heard the term tritone substitution and I looked it up on Wikipedia and here.
If I understand correctly, the tritone substition of G7 is Db7 in C. Can we consider the Db7 chord as being a G ninth dominant chord, with a flatted fifth and a minor ninth but with missing root, i.e.
B-Db-F-Ab to fit for four-part harmony?
So, can the Db7 chord people talk about enharmonically be the same with
B-Db-F-Ab with a missing root (and with all the proper inversions)?
Last but not least, can the tritone substitution chord modulate up a tritone? For example, can the Db7 in C as a substitution chord of G7 be resolved in Gb and end the phrase in Gb?