Tritone substitution: D7 can be replaced by G#7 (a tritone away). Both chords share the same tritone between the 3rd and 7th, resolving to the same tonic.
The minor 9th forms a compound-semitone dissonance with the root, while the minor 7th preserves dominant function. On V in minor, b9 normally corresponds to scale degree 6 and resolves downward.
Application
V7b9 → i in minor
Flamenco and Latin jazz
Minor jazz cadences
Voicing & structure
Root position
v1D(D4)Bass
root
v2F#(F#4)M3from D
v3A(A4)m3from F#
v4C(C5)m3from A
v5Eb(Eb5)m3from C
Harmony
Compatible scales
All chord tones are contained in these scales — use them for improvisation.