C# addb9/C
The C# addb9 chord consists of the C# (Perfect unison), F (Major third), G# (Perfect fifth) and D (Minor ninth) notes, with C as the bass note.

How to play the C# addb9/C chord
- Play C as the lowest bass note in the left hand
- Find C# (Root) on the piano keyboard
- Add F (Major third) — 4 semitones above C#
- Add G# (Perfect fifth) — 7 semitones above C#
- Add D (Minor ninth) — 13 semitones above C#
- Press all 5 notes simultaneously to sound the C# addb9/C chord