A 13/A#
The A 13 chord consists of the A (Perfect unison), C# (Major third), E (Perfect fifth), G (Minor seventh), B (Major ninth) and F# (Major thirteenth) notes, with A# as the bass note.

How to play the A 13/A# chord
- Play A# as the lowest bass note in the left hand
- Find A (Root) on the piano keyboard
- Add C# (Major third) — 4 semitones above A
- Add E (Perfect fifth) — 7 semitones above A
- Add G (Minor seventh) — 10 semitones above A
- Add B (Major ninth) — 14 semitones above A
- Add F# (Major thirteenth) — 21 semitones above A
- Press all 7 notes simultaneously to sound the A 13/A# chord