^Well, in this case:
In the CMaj9 chord:
You beat me to it. Beepster = Roadrunner? lol)
This next portion has to do with voice-leading which is also essential to getting good harmony. Probably referred to as Part Writing in Theory classes! (I had a hard time, too lol)
Two voices can be a fifth apart from each other, but that interval can't be present as they voices progress from chord to chord.
This progression: CMaj7-Dm7-Em7 (a rootless CMaj9)
The bass could walk from C-E or he could walk up then walk back down (C-D-C)
the Soprano could follow him (walk up E-G) or (E-F-E) or walk down (G-F-E; contrary motion)
Then, you have the inner voices:
The tenor could walk up from G-B; walk down B-G; or GAG himself as can the alto; however, you have to keep in mind the interval relationship between adjacent voices.
If there is a parallel fourth, fifth or octave within the relationship found anywhere in the progression once it starts, then something has to be changed.
It'll take alot of practice to get this. A great way to practice is to play series of chords using inversions.
C-Am-F-G (I-vi-IV-V)
C in second inversion (G-C-E) , Am in root position (A-C-E), F in first inversion (A-C-F) and G in first inversion (B, D, G) This allows you to walk up to C in root position (C-E-G).
Of course, if I were to add the fourth voice:
C in second inversion (G-C-E-*C) , Am in root position (A-C-E-*C), F in first inversion (A-C-F-*A) and G in first inversion (B, D, G-*B) This allows you to walk up to C in root position (C-E-G-*C).
* = Soprano note
The outer voices - bass went G-A-A-B-C; the soprano went C-C-A-B-C
The inner voices - tenor went C-C-C-D-E; the alto went, E-E-F-G-G
That is an example of good voice-leading - not to mention there
being as little movement as possible with the hands and fingers which is what is being asked when talking about the voices and how they're supposed to move.
This is of course, referring to choral music and while this guideline can be broken, if you pretend your fingers were those voices you realize how such should be followed.