Mac,

My theory books give Cdim7 as C Eb Gb Bbb (which translates as C Eb Bb A).

Most of the books usually explain the term 'diminished' using the following: the harmonic intervals C to B = major 7th; C to Bb = minor 7th; C to Bbb = diminished 7th. That is, for an interval to be diminished, it's 2 semitones lower than the major interval and 1 semitone lower than the minor interval.

The exception to the above occurs when perfect intervals are used (e.g. perfect 4th, perfect 5th and perfect 8th). In one of the 'perfect' intervals, there are no minor intervals and the term diminished means "one semitone lower than perfect". Thus C to Gb is a diminished 5th and E to Bbb is also a diminished 5th. My understanding of the diminished 7th chord is that much of its power comes from the fact that it contains two diminished 5th intervals.

Classically, a diminished 7th chord usually resolves by contracting the two diminished 5th intervals to two intervals of a 3rd (major or minor) without worrying about what doubling occurs.

Noel


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