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The French sixth chord sounds more dissonant than the Italian and German sixth chords. What is the reason?

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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#

Ab to D and C to F# are both tritones.

By contrast, the Italian and German versions would only have one: C to F#.

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    So, it is like a diminished 7th(which is also 2 overlapping trritones) but based on the major chord instead? – Caters Feb 13 at 17:47
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    @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 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! – Maika_Sakuran0miya Feb 20 at 5:15

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