Hughes
The only exception is the double tone F#-G♭, which is a curious study. F# as a harmony takes the double tones as sharps, and F♮ is E#. G♭ is also a harmony sounding the same tones, by taking the double tones as flats, and B♮ as C♭. F# therefore takes the imperfect tone of E#, and G♭ the imperfect tone of C♭. (See here the harmony of G♭ in musical clef.) [Harmonies of Tones and Colours, Combinations of dissonance, rests, page 24]