Two link questions

I have two questions about links.

1. The most common link pattern is the one-to-one link. If I got it right,
there's no need to separate one-to-many from many-to-one, because that's
controlled by the link's direction. How about many-to-many? Do you ever
need a many-to-many link?

2. I'd like to know more about link-bases, but it seem so elusive and XLink
doesn't say much about it. Is there a good place to start?

Gustaf