The French sixth chord sounds more dissonant than the Italian and German sixth chords. What is the reason?
It's because the French 6th chord has an extra tritone.
If we take an Ab Augmented 6th chord, we would have:
Ab C D F#
F# are both tritones.
By contrast, the Italian and German versions would only have one:
1So, it is like a diminished 7th(which is also 2 overlapping trritones) but based on the major chord instead?– CatersFeb 13, 2019 at 17:47
1@Caters I've never thought about it that way. It's true that they both contain two tritones, but their harmonic function is different enough that I'm not sure it is a fruitful line of thinking in any deep sense. But I suppose it does explain why they have similar amounts of dissonance. (The whole step in the FrAug6 is why it sounds even clashier than then dim7)– Ben I.Feb 13, 2019 at 20:08
Ah, now I got it! While the Italian and the German sixth chords have only one, the French sixth has TWO tritones, the A4 / d5 intervals, which are EXTREMELY dissonant. The tritones are known as "Devils", which were banned in early church music!– user53472Feb 20, 2019 at 5:15