°7 chords
Notated as °7, e.g. C°7.
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. C°7 = C Eb Gb Bbb = C Eb Gb A.
A °7 chord is actually just the top 4 notes of a 7b9 chord a M3 below it. Thus, a °7 chord has dominant function.1
e.g. C°7 is the upper part of Ab7b9.