Hi, Moderator and Bob,

I've found something in "GLOSSARY - iReal Pro" thread:

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03-09-2013, 05:34 AM #4 pdxdjazz
Diminished, Diminished 7th, Half-Diminished
The diminished seventh chord contains the further diminished 7th (enharmonicly the 6th) so that all the notes have a step and a half (minor third) between them.
Co=Co7 contains: C,Eb,Gb,Bbb(A)
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I believe Co is not the same as Co7 now.
Co = C, Eb, Gb
Co7 = C, Eb, Gb, Bbb(A)

# if my memory serves me right, Co used to be the same as Co7 before, though.