º7 chords
Notated as º7, e.g. Cº7.
These chords are particularly dissonant. They’re comprised of the 1 b3 b5 bb7, enharmonically 1 b3 b5 6. There’s 2 tritones in them!
e.g. Cº7 = C Eb Gb Bbb = C Eb Gb A.
A º7 chord is actually just the top 4 notes of a 7b9 chord a M3 below it. Thus, a º7 chord has dominant function.
e.g. Cº7 is the upper part of Ab7b9.