A 7b9 Chord
How the A 7b9 chord is built: its notes, its interval structure, and where it fits.
The A 7b9 chord contains 5 notes: A – A# – C# – E – G. It belongs to the altered family of chord qualities. Like every chord, it is defined by the distances (intervals) between its notes, not by any one instrument — which is why the same formula names the chord whether it is voiced on a fretboard, a keyboard, or a choir.