2. Manual of Classification
A numerical arrangement of patent subjects by class and subclass. Subclasses are arranged in hierarchical outline format; find your subclass number, examine it and the other subclasses indented underneath it and choose the subclass(es) that best describe your invention.
If you are having trouble understanding the classification, you may read the Examiner's Handbook (www.uspto.gov/web/offices/pac/dapp/sir/co/examhbk/) on the Use of the U.S. Patent Classification System. You may also submit a written request for help in finding appropriate classification numbers to the U.S. Patent and Trademark Office; forms are available at the Science Reference Desk.
Paper: Science Section - Patent, Trademark & Copyright Collection
DVD ROM: CASSIS Patents ASSIST (Tools)
Internet: Manual of Patent Classification (www.uspto.gov/go/classification/selectnumwithtitle.htm)