Tritone substitution: F#7 can be replaced by C7 (a tritone away). Both chords share the same tritone between the 3rd and 7th, resolving to the same tonic.
The augmented 11th forms a tritone with the root and avoids the close clash that a perfect 11th creates with the 3rd. The combination of major 3rd, minor 7th, and #11 matches the Lydian dominant scale.
Application
Lydian harmony
Jazz and fusion
Film scoring
Voicing & structure
Root position
v1F#(F#4)Bass
root
v2A#(A#4)M3from F#
v3C#(C#5)m3from A#
v4E(E5)m3from C#
v5G#(G#5)M3from E
v6B#(B#6)M3from G#
Harmony
Compatible scales
All chord tones are contained in these scales — use them for improvisation.