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

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