The standard grips this chord is usually shown with.
Cm9 is C, Eb, G, Bb and D — Cm7 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.

| C | Eb | G | Bb | D |
|---|---|---|---|---|
| 1 | ♭3 | 5 | ♭7 | 9 |
Also written Cmin9, C-9 or C minor 9. The same chord either way; which spelling you meet depends on who wrote the chart.
Cm9 in 1st inversion puts Eb in the bass instead of C, the 3rd of the chord. The notes are the same; what changes is what the bass line does, which is most of why you would reach for it — it lets the lowest voice move by step where the root would have jumped.
2 of Cm9’s 9 common shapes do this:x6878811-13-12-12-13-11
Play Cm9 as a shell — root, third and seventh — and the spare string takes the 11 (F) or the 13 (A).
All 9 common shapes as fret numbers, lowest string first. Hover or tap one to see it drawn.
How you get to it. The turn that lands on Cm9 is Dm7♭5 to G7♭9: the ii and the V of C minor, which is where a minor tonic is usually arrived at rather than simply begun on.
In which keys. Every note of Cm9 belongs to C minor (the i, the tonic), Eb major (the vi, the submediant), G minor (the iv, the subdominant) and Bb major (the ii, the supertonic).
Every note of Cm9 is in C dorian and C natural minor.
Change one note of Cm9 and you have Cm69 (the flat seventh becomes the sixth), Cm7 (drop the ninth), Cm11 (add the eleventh), CmMaj9 (the flat seventh becomes the major seventh), Cmadd9 (drop the flat seventh) or C9 (the minor third becomes the major third).
Cm9 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.