1. http://www.wipo.int/classifications/ipc/en/.
2. http://www.stn%96%3binternational.de/fileadmin/be_user/STN/pdf/presentations/0910ClassOnClass.pdf.
3. http://www.espacenet.com/getstarted/classification_search_en.pdf.
4. http://www.wipo.int/treaties/en/ShowResults.jsp?lang=en&;treaty_id=11.
5. http://worldwide.espacenet.com/help?topic=classesqh&locale=en_EP&method=handleHelpTopic.