For novice patent searchers one of the most difficult concepts in the U.S. Patent Classification system is the idea that inventions can be classified on the basis of "proximate function." Proximate function is one of four schemes in the USPC used to classify subject matter disclosed in patents and published applications. The other three are "industry or use," "effect or product" and "structure." The rationalie behind proximate function is that "similar processes or structures that achieve similar results by the application of similar laws of nature to similar substances are considered to have the same fundamental utility and are grouped together." (See the Handbook of Classifiction.) For example, a refrigeration system used to cool beer and a refrigeration system that cools milk are treated the same under the USPC.
The patents of A.C. Gilbert, inventor of the Erector Set, offer another example of proximate function at work. Gilbert's inspiration for the Erector set was real-life construction sites he saw around his home in New Haven, Connecticut. Many of his patented inventions are simply scaled-down versions of construction materials such as beams, girders, brackets, trusses and rivets. Consequently, you can find Gilbert's patents classfied under both Class 446, Amusement Devices: Toys, and Class 52, Static Structures (Buildings). Because of the concept of proximiate function a search for patents for construction toys should include the appropriate subclasses from both classes. A third search possibility is the design class D21, Games, Toys and Sports Goods; subclasses 484-505 specifically relate to construction-type toys.
I'm the librarian for research services in the Engineering and Science Library at Queen's University in Kingston, Ontario. I've been working with patent information since 1991, including seven years at the U.S. Patent and Trademark Office. I believe that the dissemination of patent information is a public good and should be promoted, especially in the education of science and engineering students.