A dim7
The A dim7 chord consists of the A (Perfect unison), C (Minor third), D# (Diminished fifth) and F# (Major sixth) notes.

How to play the A dim7 chord
- Find A (Root) on the piano keyboard
- Add C (Minor third) — 3 semitones above A
- Add D# (Diminished fifth) — 6 semitones above A
- Add F# (Major sixth) — 9 semitones above A
- Press all 4 notes simultaneously to sound the A dim7 chord