C 11/C#
The C 11 chord consists of the C (Perfect unison), E (Major third), G (Perfect fifth), A# (Minor seventh), D (Major ninth) and F (Perfect eleventh) notes, with C# as the bass note.

How to play the C 11/C# chord
- Play C# as the lowest bass note in the left hand
- Find C (Root) on the piano keyboard
- Add E (Major third) — 4 semitones above C
- Add G (Perfect fifth) — 7 semitones above C
- Add A# (Minor seventh) — 10 semitones above C
- Add D (Major ninth) — 14 semitones above C
- Add F (Perfect eleventh) — 17 semitones above C
- Press all 7 notes simultaneously to sound the C 11/C# chord