Call it what you want, a C ninth chord is a C major chord and a G minor chord sounding together--where you see a C9 written, you can often play Gm, or play a run based on a Gm chord--

the fact that these messy big chords can be broken down into simple triads is a very useful thing to know about when you are working out chord substitutions to get the right sound with chords that make sense on a guitar--