F b5/C#
The F b5 chord consists of the F (Perfect unison), A (Major third) and B (Diminished fifth) notes, with C# as the bass note.

How to play the F b5/C# chord
- Play C# as the lowest bass note in the left hand
- Find F (Root) on the piano keyboard
- Add A (Major third) — 4 semitones above F
- Add B (Diminished fifth) — 6 semitones above F
- Press all 4 notes simultaneously to sound the F b5/C# chord