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Introductory information on patents and patent searching.

Types of searches

This part of the guide refers specifically to U.S. patent sources, but the strategies listed will work when searching for international patents as well. There are three search techniques that are especially useful when searching in the patent literature. Scroll down or follow the link to read a description of each.

Thorough searches will include all three techniques. A common strategy begins with a keyword search, which is used to identify relevant patents. The classification numbers and cited references used in the patents identified are then investigated to find more patents and information. As with all research, patent searching is an iterative process, using the information found in one search to identify new areas to research.

Keyword Searching

A keyword search is a great place to start patent research. However, keyword searching in patent databases can be fairly complex. Patent language tends to be either technical or legal in nature, but is not standardized. Any form or synonym of a word can and probably is used. In addition, most inventions have several aspects and facets worth exploring. It is important to create a robust set of keywords before beginning your search. The following questions are useful in helping to generate keywords based on the different aspects of an invention:

  • What problem does the invention solve?
  • What is the invention? What is it made of?
  • What does the invention do?
  • What is the end result?

Once you have answered these questions, get a thesaurus and write down synonyms for the answers to each question. Also try to think of equivalent technologies and broader and narrower terms.

You can begin your keyword search in any of the databases listed under Patent Databases and Agencies. It is a good idea to read the description of each database and think about your needs when choosing which database you will start with. If you want to try to search full text before 1976, try Google Patents. If you are looking for more recent technologies Free Patents Online or the USPTO database will work well. I find Free Patents Online or Google Patents to be good places to start, keeping in mind that you should always check in the USPTO database for the most current version of any document you identify in Free Patents Online or Google Patents.

If you are looking for foreign patents, please refer to the International Patents page to select a database to search in. esp@cenet is an excellent place to start a foreign patent search.

Classification Searching

The patent classification system is used to organize patents into groups of similar technologies. When a patent is granted, the examiner assigns main and related U.S. class numbers to the patent. The patent examiner may also include an International Class number and the class numbers used to search for prior art. U.S. classification numbers take the form CCC/SSS where CCC is the three digit class number and SSS is the three digit subclass number. 

Often a classification search begins by using a keyword search to find relevant patents, then looking up the classification numbers assigned to those patents in the Classification Schedule. In the HTML version of the patent in the USPTO database, the class numbers appear below the filing date, as in the snippet below for the Toy Bar Soap Slide (#5,078,642). In the actual patent document, the class numbers are fields (51) International class number, (52) US class number, (58) Field of search.

If you have a general idea of the field of the invention, such as computer, motor vehicle, toy (as in the Toy bar soap slide), etc. you can look it up in the Index to the United States Patent Classification System. Click on the letter of the type of invention, such as T for Toy, then use CTRL +F to search for the word Toy.

You can also try using the search box available at the Classification Main Menu page.

For more information and frequently asked questions about the U.S. Patent Classification website, visit the Help File.


Toy bar soap slide


Inventors: Glessner; Jon L. (Columbus, OH)
Appl. No.: 07/602,812
Filed: October 24, 1990
Current U.S. Class: 446/153 ; 4/546; 4/559; 446/168; 446/177
Current International Class: A63F 7/00 (20060101); A63F 7/38 (20060101); A63H 18/00 (20060101); A63H 18/02 (20060101); A63H 023/00 (); A61H 033/02 ()
Field of Search: 446/153,168,170,173,176,444,92,177 272/56.5R,1B 4/538,591,546,628,559 248/683,309.3,309.2,309.1,362,363,317,318

Citation Searching

Every patent includes a list of References Cited. This is usually a list of related patents and sometimes includes references to journal articles or other scholarly literature. When viewing the HTML version of a patent, the patent references usually appear as links so you can easily click through to read the references.

Similarly the HTML version of a USPTO patent includes the Referenced By link to all of the patents which cite the patent you are currently viewing.

If you know the patent number of a technology of interest and the patent is dated after 1976, a keyword search for the patent number will also return all the patents which cite your patent.

Citation searching is an excellent way to track trends in a particular field or technology.