Ok, this is probably an easy one, but I could swear I have read it in one or the other Active Directory related book, and now I just can't get it out of my head even though no other sources seem to confirm it, and it sounds unnecessary and wrong. It could easily be from some documentation dating back to very early betas of AD, for all I know.
The statement was that when you create a child domain (say "accounting.france.company.lcl") of a domain (say "france.company.lcl")that is itself already a child domain of the tree-root domain(say "company.lcl"), Active Directory will not only create a two-way transitive trust between these two parent and child domains, but also an automatic two-way transitive trust directly between the downlevel child, and the tree-root 2 steps up. So the trust between the downlevel child domain and the tree-root domain would not only be implicit - transitive through the intermediate direct parent domain - but also more like a shortcut trust.
It would just be nice if someone could wipe this out of my head by a strong statement. :-)