F# / Gb

F#/Gb is the tritone — exactly halfway through the octave from C. This symmetrical position gives it a restless, unresolved quality that is central to dominant seventh chords and blues harmony.

The tritone (F# relative to C) was called "diabolus in musica" (the devil in music) by medieval theorists who considered it too dissonant for sacred music. Today it drives the tension in dominant seventh chords and is the backbone of blues and jazz harmony.