A Dominant Seventh Flat Nine chord (A7b9) on the guitar fretboard

The A Dominant Seventh Flat Nine chord (A7b9) is made of A (the root), C# (major third), E (perfect fifth, optional), G (minor seventh) and Bb (minor ninth). The fretboard shows every one of them across the neck in standard tuning (E A D G B E), and plays them back so you can hear the chord.

The seventh flat nine adds a lowered ninth to the dominant seventh, a semitone above the root. On the neck the root and the flat nine sit side by side, and the fifth is optional. It is the tense sibling of the plain ninth chord.