How this term is used here In this channel's material, A-flat add nine appears as a concrete musical example in the video Codex Taught Me How to Play Piano. That video shows how to connect a MIDI piano to a computer and have Codex build a small app that displays what you're playing — turning casual noodling into a way to explore and understand specific chords, songs, and techniques. A-flat add nine is one of the chords explored in that hands-on, AI-assisted piano setup. Evidence for its use is limited to this context; no other material in the captured set defines or discusses the chord in depth.