Hello, I know that there is a cardinal rule that you should never link between your own sites lest be banned by goog. Is this true, and if so, how does goog know?
No its not true. you will not get banned but only think is that google will give you less weight as if your both sites are on same Ip
I agree that GG may give less weight to the links. It only makes sense that some sites that you own will be linked together. For instance: many large corp have many sub companies that are synergistic. It would make sense for them to be linked. Sometimes companies run print ads that talk about many products or many divisions. It makes sense and no one gives it much thought. That is my point: If it makes sense for your business, it's a good thing to do. Sometimes if I want to run an ad from EVERY page of a site to another site I'll use a nofollow tag so I'm not spamming my own site.