C#m9 is C#, E, G#, B and D# — with the ninth stacked above it.
It sounds warm, and considerably richer than an m7. The standard voicing for a ii chord once the plain one has been played enough.
Pre-dominant or minor tonic — an m7’s job with more colour.
Also written C#min9, C#-9 or C# minor 9.
Jazz players rarely leave a ii–V–I as bare sevenths: C#m9, F#13, Bmaj9. The ninth on top of the ii is already the V’s thirteenth, so it holds over the barline before falling a step to the ninth of the I.
Listen for: The ninth holds as the V’s thirteenth, then falls a whole step. On the plain m7 that string starts on the root and has to climb.
Leave the root off C#m9 and the four notes left are Emaj7. So the I can turn into the vi without moving a finger above the bass, a tonic swap jazz players make freely.
Listen for: Nothing above moves. A bass note arrives a minor third below the I’s root, and the chord is minor.
In which keys. Every note of C#m9 belongs to C# minor, E major, Ab minor and B major.
Every note of C#m9 is in C# natural minor and C# dorian.
Each of those is a page: the scale across the neck, and the chords it harmonises into — which is where C#m9 sits in it. Every scale.
Change one note of C#m9 and you have (the flat seventh becomes the sixth), (drop the ninth), (add the eleventh), (the flat seventh becomes the major seventh), (drop the flat seventh) or (the minor third becomes the major third).
C#m9 is a minor chord — root, minor third, fifth. Minor chords on every root.
The common shapes and the derived voicings come from different places, and both are explained in how this site is built.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.