Tritone ratios

Graham Breed wrote:
> No, the tritone is not always the same pitch. It can be 7/5, 10/7,
> 25/18 or 36/25. The first two do not arise in a diatonic context.
> So, 25/18 is an augmented fourth and 36/25 a diminished fifth. The
> difference between them, 63 cents, is not negligible.

I had always assumed that 7/5 was the natural ratio for an augmented fourth,
and that 10/7 was the natural ratio for the diminished fifth. What do you
mean when you say they "do not arise in a diatonic context"?

I generally use Partch's "Genesis" as a reference to convert ratios to
cents, and I was surprised that I couldn't find 25/18 and 36/25 in his
charts. Can anyone shed some light on this?

-b0b-

// http://wco.com/~quasar/articles/just_e9.html


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