Roger Mills wrote: > Given a tritone, if you raise the top note 1/2 step and lower the bottom tone 1/2 step, you resolve to a major chord; this is because, of course, the tritone contains 2 of the 4 notes of a Dom7, which of course resolves to Tonic. And, of course, the chord of the diminished seventh, composed of two interlocking tritones (e.g. C-Eb-F#-A), was a standard means of modulation in the 18th-19thC for exactly that reason - drop any note by a semitone and you have a complete dominant-seventh chord. Other chords containing two tritones either form part of a whole-tone scale (C-D-F#-G#) or contain scrunchy semitones (C-C#-F#-G). Debussy used the first (Pelleas, act 3 scene 2-3 if memory serves), and I'm sure somebody's used the second - Ligeti or Webern maybe? Jonathan.