Tritone substitution: G#7 can be replaced by D7 (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
v1G#(G#4)Bass
root
v2B#(B#5)M3from G#
v3D#(D#5)m3from B#
v4F#(F#5)m3from D#
v5A(A5)m3from F#
Harmony
Compatible scales
All chord tones are contained in these scales — use them for improvisation.