A dim
The A dim chord consists of the A (Perfect unison), C (Minor third) and D# (Diminished fifth) notes.

How to play the A dim chord
- Find A (Root) on the piano keyboard
- Add C (Minor third) — 3 semitones above A
- Add D# (Diminished fifth) — 6 semitones above A
- Press all 3 notes simultaneously to sound the A dim chord