Originally posted by Christoph

What about diminished 7ths? I remember you tried to explain diminished chords to me a long time ago. A diminished 7th interval is the same as a 6th, right?

Yes. Technically there is no such thing as a diminished 7th interval, 7th intervals are only major and minor. So diatonically the diminished 7th chord is composed of the root, minor 3rd, diminished 5th, and 6th scale degrees. The tertian method of labeling chords though notates the 6th scale degree as a double flatted 7th.