Also called half-diminished chords.
Formula 1 b3 b5 b7, so Cm7b5 would be C Eb Gb Bb, i.e. 8643xx.
Commonly used as the II chord in a minor II V I progression, e.g. Dm7b5 G7b9 Cm7.
Here are the five CAGED shapes for m7b5 chords -
Cm7b5: x31312
Am7b5: x01213 or x01013
Gm7b5: 31x021 (virtually unplayable, better is 3xx321)
Em7b5: 012030 or 012033
Dm7b5: xx0111
And here are all the m7b5 chords I can think of using open strings in low positions -
Am7b5: x01213 or x01013
Bbm7b5: x12120
Bm7b5: x20231
C#m7b5: x42000
Dm7b5: xx0111
Em7b5: 012030 or 012033
Fm7b5: 1x1101
F#m7b5: 202212 (use thumb to fret 6th string)
Gm7b5: 31x021
G#m7b5: 4x0102
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IMO the moveable A and D -shapes are most useful
Seriously: If you value it, take/fetch it yourself