10.20Tritones recur every dominant chord

D7:

D F♯ A C

guide tones:

F♯ + C

resolve:

F♯ → G

C → B

to G.

A7:

A C♯ E G

guide tones:

C♯ + G

resolve:

C♯ → D

G → F♯

to D.

Same mechanism.