Using US Patent Classifications to Enhance Key Word Searching to Achieve Higher Quality Patent Search Results

Over the last year, while building ArchPatent, I have spent a lot of time speaking with inventors and lawyers about various aspects of prior art research, and I learned a few great tricks along the way which I wanted to share. In particular, I wanted to write this article to explain how a key word patent search can be enhanced by filtering the results using US classifications and subclassifications. First, I’ll provide some background about the U.S. Patent Classifications Systems (USPC), discuss how filtering key word search results by US classifications and subclassifications fits within the seven step prior art search strategy outlined by the USPTO and provide specific instructions for how this type of search can be conducted to find highly useful results and save significant time over the current manual process.

Overview of US Classification and Subclassifications

In June of 2011, The USPTO published a helpful 15 page report titled, “Overview of the U.S. Patent Classifications Systems (USPC).” This report provides a very thorough overview for anyone seeking a deep dive into the US Classifications system.

I have found it helpful to think about patent classifications as being large buckets with subclassifications being little buckets within the larger classification buckets. There are currently more than 450 classifications and over 150,000 subclasses.  According to the USPTO, “A class generally delineates one technology from another. A Subclass delineates processes, structural features, and functional features of subject matter encompassed with the scope of the class.” It is also important to realize that although every patent has only one primary classification (class and subclass) it may include others as applicable to the patent.

It is also important to realize that over time classifications and subclassifications are created, deleted or merged. The result of this is that sometimes scores of patents have their classification changed. Although all similar types of patents should have similar classifications and be grouped together, they may not be especially true for emerging technologies that overtime may require a new class or subclass. Everything is classified by an examiner, so any given patents may not necessarily be where you would think it should be.

The USPTO Manual of Classification (MOC) includes a number of ways to search within the classes and subclasses. To help visualize the relationships between classes and subclasses, we created a USPTO Classification Tree on the ArchPatent site that lets you drill down for each class into its associated subclasses. This type of resource can greatly simplify classification research.

How US Classifications Fit Within the USPTO Seven Step Prior Art Search Strategy

The USPTO provides a Seven Steps Prior Art Search Strategy to be used when conducting prior art research. These steps are outlined below.


1. Brainstorm keywords related to the purpose, use and composition of the invention.

2. Look up the words in the  Index to the U.S. Patent Classification to find potential class/subclasses

3. Verify the relevancy of the class/subclasses by using the  Classification Schedule in the Manual of Classification.

4. Read the  Classification Definitions to verify the scope of the subclasses and note “see also” references.

Access Full Text

5. Search the Issued Patents and the Published Applications databases by “Current US Classification” and access full-text patents and published applications.

Review and References

6. Review the claims, specifications and drawings of documents retrieved for relevancy.

7. Check all references and note the “U.S. Cl.” and “Field of Search” areas for additional class/subclasses to search.

After completing step #1, I believe that it is possible to tackle prior art research in a much more efficient and effective manner utilizing the results of multiple key word searches to drill down into relevant classifications and subclassifications. Basically this allows you to combine #2, #3. #4 and #5.

How to Use the US Patent Classifications to Enhance Key Word Searching to Achieve Higher Quality Prior Art Search Results

The first step is the same as the USPTO’s Seven Steps Prior Art Search Strategy. Start by brainstorming “keywords related to the purpose, use and composition of the invention.”  Let’s say you are interested in searching for patents related to the design and manufacture of golf ball coverings.  We would start by searching for “golf balls”.  At the time of this writing, these keywords will return 21141 results on ArchPatent.  Looking at the filter rail on the left side of the results page, you will see a listing of classifications by name, ordered by most hits within the search results.  The first classification listed is “Games Using Tangible Projectiles,” which means that a majority of the results containing the key words “golf balls” fall within this classification.  Clicking this link will filter the search results to patents that contain the term “golf balls” and have the classification ”Games Using Tangible Projectiles”.

So now we know how golf ball patents are classified.  By clicking the arrow next to the class, you will see there is a subclassification titled “Golf”.  By loading the full classification tree, which is a link in the bottom of the US classification list, you can then drill down even further.  If you notice, there is a subclassification for golf balls, and there are 2087 golf ball patents.  Examing the subclasses under “Golf” reveals a subclassification “Particular cover (e.g., size, material, dimple pattern, etc.)”, which has a subclassification number 473/378000.  And now you see that there are 447 patents on golf ball covers.  Hence, you now have a focused set of patents regarding golf ball covers, and your prior art analysis starts here.

Filtering Classifications and Subclassifications by Examiner

As you work through the classification and subclassification review for each key search term, it may be helpful to also search the results by “Examiner”. Patent Examiners often work at the US Patent Office in a specific area of expertise for many years. This filter allows you to see very Patent Examiner that has reviewed patents and the number of patents they have reviewed.

Filtering Classifications by Most Referenced

Similar to the “Examiner” filter it also is helpful to see what patent is most referenced by other patents within a classification or subclassification. Similar to a family tree, it is often helpful to see what patents examiners site the most often when granting patents within a classification or subclassification.


I hope you have found this article to be insightful. With over 150,000 subclassifications, I have found this approach to help cut down the number of hours spent gathering information and allow for a great deal more time for analyzing the results.


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Join the Discussion

5 comments so far.

  • [Avatar for Brad]
    February 3, 2012 01:24 pm

    Thank you for the helpful comments. I plan to do some follow up research based on your insightful thoughts.


  • [Avatar for Stan Schwartz]
    Stan Schwartz
    January 28, 2012 12:51 pm

    I have been searching patents utilizing United States classifications in the manner you suggested. What is very important and not mentioned in your article, is the importance for utilizing the very helpful notes and rules of classification set forth in Class Definitions. This is particularly true in many technologies, especially chemical technologies where there appears to be some overlap in subject matter coverage among classes. Understanding which class is superior for claimed subject matter can make a major difference in search results.

  • [Avatar for oldguy]
    January 28, 2012 11:09 am

    It is interesting to note that the EPO allows for the examiner to add certain class selected terms to the search field of the issued patent. For example should the applicant use terms like “pressure pad” or “friction member” and never use the term “clutch” the examiner can add that term to the search field. As I understand it, currently the US system does not allow that.

  • [Avatar for EG]
    January 27, 2012 09:43 am


    Thanks for writing and posting this article. Havin grown up searching the “old fashioned hard (copy) way” (i.e., searching through the “no longer in existence” patent shoes), I’m very familiar with the Manual of Patent Classification. And I frequently use class/subclasses in combination with word search criteria to refine the search, especially when the word search alone gives you over 500 hits.

  • [Avatar for ODP]
    January 27, 2012 08:27 am

    Does anyone with enough experience in patent law to understand this blog really not understand how to use the patent classification system? What’s a 102(b) while you’re handing out obvious (useless?) information so freely?

    Another great article from the web’s self-proclaimed #1 patent blog.