C# 7b9 (C#7b9)
Also written as Db 7b9 (Db and C# are the same key).
Notes in C# 7b9
Inversions
Same notes as C# 7b9, with a different chord tone as the lowest note. Useful for smoother movement between chords instead of jumping around the keyboard.
Root position
C# in the bass
1st inversion
E# in the bass
2nd inversion
G# in the bass
3rd inversion
B in the bass
4th inversion
D in the bass
Open voicings
A close-position chord like C# 7b9 can sound cluttered in the middle of the keyboard. Jazz players open it up by dropping one note down an octave, spreading the same notes wider without changing the chord.
Drop 2
B on the bottom
Drop 3
G# on the bottom
Play C#, E#, G#, B and D together and you get C# 7b9.
A dominant seventh with a flattened ninth stacked on top, tense and dramatic. It shows up constantly in jazz and in flamenco-adjacent harmony, and it resolves with even more urgency than a plain dominant seventh does.