Patent Convention - available in either html or pdf versions from the
European Patent Office (EPO) site.
How to search classes - from the University of Washington Library site.
International Classifications - from the World Intellectual Property Organization (WIPO) site
Manual of U.S. Patent Examining Procedure - in hypertext format, prepared from the USPTO version by Bill Goodwine at Notre Dame University, USA.
U.S. Classes - From the USPTO site
U.S. Patent Classification - Classes by Title - prepared by Michael White, Librarian for Research Services, Queen's University, Canada