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

How to play the C addb9/G# chord
- Play G# 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 C# (Minor ninth) — 13 semitones above C
- Press all 5 notes simultaneously to sound the C addb9/G# chord