C b5/A#
The C b5 chord consists of the C (Perfect unison), E (Major third) and F# (Diminished fifth) notes, with A# as the bass note.

How to play the C b5/A# chord
- Play A# as the lowest bass note in the left hand
- Find C (Root) on the piano keyboard
- Add E (Major third) — 4 semitones above C
- Add F# (Diminished fifth) — 6 semitones above C
- Press all 4 notes simultaneously to sound the C b5/A# chord