F#7b5

— About this chord

F#7♭5 (1–3–♭5–♭7) flattens the 5th of a dominant 7th for an unsettled, whole-tone-adjacent tension often heard in bebop reharmonizations and tritone-substitution turnarounds. It sits at the crossroads of dominant and diminished harmony, giving comping guitarists a compact way to add bite between diatonic chords. On guitar it forms a very tight shape with the flatted 5th just a half-step from the root, and slides easily up the neck as a movable form without changing fingering. Functionally it often substitutes for a plain F#7 en route to B, or acts as a tritone-related dominant borrowed from C7, coloring ii-V-I cadences with extra edge. Also searched as F#dom7b5, Gb7b5, or G♭7♭5.