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

How to play the C dim/A# chord
- Play A# as the lowest bass note in the left hand
- Find C (Root) on the piano keyboard
- Add D# (Minor third) — 3 semitones above C
- Add F# (Diminished fifth) — 6 semitones above C
- Press all 4 notes simultaneously to sound the C dim/A# chord