Originally Posted by Andrew Dee
As a 'less talented' guitarist, I learned to play Bdim as per the attached - but it has an Ab/G# in it i.e. B, D, F, Ab - what is the true name of that? Is that the dim7?
Then a few years after, I found out there was a chord called Bm7b5 (B, D, F, A) (attached) which sounds slightly less confrontational.
Andrew
These are respectively the diminished (sometimes for clarity fully-diminished) and the half-diminished. The former usually indicated by the 'degrees' symbol, the latter by a similar symbol with a diagonal line through or by the m7b5 form.
In the former, the 7 is doubly flattened relative the Maj7, in the latter singly flattened. (e.g. A#->A->G#)

For just the triad, people often write the ordinary 'degrees' symbol, which is OK for the triad, but can be misleading if one is using 7ths in voice leading.

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