Q: This gets even more complex when it gets to 9ths etc.
A: Cmaj9 includes maj3, and maj7, and 9. so C, E, G, B, D (notes do NOT have to be in that order and probably will sound better in a different order). Cmin9 again is like Cmin7 but with D note included. if you just say C9, that would be "C dominant 9" or just C9 (again, the word dominant makes no sense but that's just what it's called. abreviated C9, and it would be C, E, G, Bb, D. again, not in that order necessarily. so, Cmaj9, Cmin9, C9, etc. are all the same as Cmaj7, Cmin7, C7, but with D note added. If you actually wanted to add the D note but NOT have the 7 in there at all, it is usually "add9". so Cadd9 would be C, E, G, D. and Cmin(add9) would be C, Eb, G, D. not necessarily in that order. it is pretty common to put the D in the same octave as the other notes. also sometimes that may be called "add2" if you prefer.