C# m9/C
The C# m9 chord consists of the C# (Perfect unison), E (Minor third), G# (Perfect fifth), B (Minor seventh) and D# (Major ninth) notes, with C as the bass note.

How to play the C# m9/C chord
- Play C as the lowest bass note in the left hand
- Find C# (Root) on the piano keyboard
- Add E (Minor third) — 3 semitones above C#
- Add G# (Perfect fifth) — 7 semitones above C#
- Add B (Minor seventh) — 10 semitones above C#
- Add D# (Major ninth) — 14 semitones above C#
- Press all 6 notes simultaneously to sound the C# m9/C chord