Hello again, twice in a day! Oh well.

As the title suggests, I'm extremely confused at this Interval, as I'm not sure when it'd ever come into play. I'm just working in the key of C Major here, to not confuse myself, but alas it's happened again.
The Diminished 7th in C Major would be Bbb (Correct, I'm sure.) But what would that be regularly? An A? An A#? How many semitones from the Root note would it be?

Also, if anybody can help me with my other thread any more it'd be greatly appreciated.
My Harmony Thread
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Its an A. Bbb is enharmonic to an A. Thats 9 semitones.

It's used in the constuction of full diminished chords and some very obscure modes (like ultra locrian).