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

How to play the C Viennese trichord/A# chord
- Play A# as the lowest bass note in the left hand
- Find C (Root) on the piano keyboard
- Add C# (Minor second) — 1 semitone above C
- Add F# (Diminished fifth) — 6 semitones above C
- Press all 4 notes simultaneously to sound the C Viennese trichord/A# chord