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

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