The Rise of the Subsite
http://www.useit.com/alertbox/9609.html

Nielsen, Jakob
Alertbox
1996

Abstract:

Web users need structure to make sense of the many and varied information spaces they navigate. The fundamental nature of the Web does not support any structure beyond the individual page which is the only recognized unit of information. For information spaces that cannot easily be hierarchically structured, the subsite can be used as a helpful additional structuring mechanism. Subsites can also be used in hierarchical information spaces to give particular prominence to a certain level of the hierarchy which is used as the subsite designator.