Tritone substitution is used for harmonic diversity
& contrast, often using altered chords.

Tritone Chord Substitution

Shown in ii - V - I - VI Turnaround

Key	ii (tritone)	V (tritone)	I	VI (tritone)
C	Dm⁷ (A^b7)	G⁷ (D^b7)	C	A^7alt (E^b7alt )
D^b	E^bm⁷ (A⁷)	A^b7 (D⁷)	D^b	B^b7alt (E^7alt )
D	Em⁷ (B^b7)	A⁷ (E^b7)	D	B^7alt (F^7alt )
E^b	Fm⁷ (B⁷)	B^b7 (E⁷)	E^b	C^7alt (G^b7alt )
E	F^#m⁷ (C⁷)	B⁷ (F⁷)	E	C^#7alt (G^7alt )
F	Gm⁷ (D^b7)	C⁷ (G^b7)	F	D^7alt (A^b7alt )
G^b	A^bm⁷ (D⁷)	D^b7 (G⁷)	G^b	E^b7alt (A^7alt )
G	Am⁷ (E^b7)	D⁷ (A^b7)	G	E^7alt (B^b7alt )
A^b	B^bm⁷ (E⁷)	E^b7 (A⁷)	A^b	F^7alt (B^7alt )
A	Bm⁷ (F⁷)	E⁷ (B^b7)	A	F^#7alt (C^7alt )
B^b	Cm⁷ (G^b7)	F⁷ (B⁷)	B^b	G^7alt (D^b7alt )
B	C^#m⁷ (G⁷)	F^#7 (C⁷)	B	G^#7alt (D^7alt )

The tritone substitution is a dominant, or secondary dominant 7th chord whose root is a tritone (3 whole steps) away from the original chord. These chords are interchangeable because the tritone interval pitches are identical in each. In Blues and Jazz turnarounds, it's common practice to use harmonic substitutions for any of the chords.