Tutorial for communication

Does anyone know of a tutorial regarding communication between objects/scenes?

For example:

I have a blank new project.

I make two movie clips (Clip A and Clip B)

I go into one of them (Clip A) and create a new layer with actionscript code in the first frame. How could I use that actionscript to refer to layers/objects in the other movie clip (Clip B)?

_root would refer to the “root” of the scene, correct? If so then I understand this, but I can’t seem to do anything further.