C# 5/A#
The C# 5 chord consists of the C# (Perfect unison) and G# (Perfect fifth) notes, with A# as the bass note.

How to play the C# 5/A# chord
- Play A# as the lowest bass note in the left hand
- Find C# (Root) on the piano keyboard
- Add G# (Perfect fifth) — 7 semitones above C#
- Press all 3 notes simultaneously to sound the C# 5/A# chord