Thread: Defining multiple classes in the same header file

What are the rules of thumb for defining multiple classes in the same header file, with the added requirement that some of those classes are member variables for other classes?

I was trying to create a linked list the other day, and my compiler kept telling me that it did not recognise some of the member variables within one of the node classes (which was a pointer to another class I had created)

Regards

EDIT: I hope this is ok to post here, as it's kinda an extension to the original Q. I didn't want to unnecessarily start a new thread