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Patent Searching: Search Methods

This guide supplements other existing guides by emphasizing patent contents and search strategies.

Basic Search Strategies

How Patents and Patent Applications are Stored

Patents and Patent Applications are stored in their respective databases using descriptive fields.  These fields contain the following:

  • Basic identifying information including:
    • Patent & application numbers;
    • Application, issue, reapplication, and other priority dates;
    • Names and locations of applicant, inventor, & assignee; and
    • Names of examiners and attorneys;
  • Content information including title, abstract, description/specification, and claims;
  • Subject information including various classification codes;
  • Patents referenced by this patent and patents referencing this patent; and 
  • Cross references to related patents or foreign applications.

Search Options

From the above discussion, one can see that the information stored about patents is analogous to information stored for scholarly works (e.g., title, abstract, author, publication date, etc.).  Thus, searching for patents can use techniques similar to those used when searching for scholarly works.  Specifically, you can search for:*

  • a known item, such as a patent number or an author (inventor);
  • keywords used in patent titles, abstracts, or full text;
  • subjects, as identified in classification codes; and
  • citations, either forward ("cited by") or backward ("references") in time.

Caution should be exercised when using keywords to search for patents because patent language is not always aligned with common language.  Sometimes titles are incredibly nondescript - e.g., "Downhole Tool" or "Valves"; other times, the language selected is more technically or legally slanted than one would normally expect to use in common discussion - e.g., "Method and apparatus of assessing downhole drilling conditions."  This is because the legal aspects of patents force a rigor on descriptive language that sometimes feels unnatural. Thus, using "subject" searches with classification codes is considered a best practice for getting the most comprehensive set of relevant results.  That said, keyword searching can be useful for identifying a starting point for your classification codes, or for broadening or narrowing your search beyond the classification codes already found.

*US patents prior to 1970 are searchable only by issue date, patent number, and current US classification

Search Strategies

When searching for patents in a particular subject area (not a known item), a recommended strategy is as follows:

  1. Brainstorm words that describe the key features of the invention/technology you are searching for (use this handy worksheet from Kari Kozak, University of Iowa);
  2. Use these words to search the classification system for classification codes most closely describing the invention/technology, and follow links to these codes; (Scroll down to see how)
  3. Look within the classification hierarchy for broader and narrower classification codes that may describe the invention/technology equally well or better; and
  4. Search for patents and patent applications that are classified according to these codes.

Note that patents and applications are often classified according to multiple classification codes.

Once a relevant patent is identified it is particularly effective to:

  • Review the classification codes assigned to that patent/application and search for other patents with any of those codes that are relevant to your subject area; and
  • Follow the citation "trail" for the patents you find; look for patents that cite them, both forward and backward in time.

If this feels like familiar territory, you are right!  These are very similar to the strategies used for searching scholarly works.

Patent Classification Systems

A Little History

Image by Emslichter from Pixabay (CC0)

Back when the world was less connected, different patent classification schemes arose, primarily including the US Patent Classification system (USPC) and the International Patent Classification (IPC), although there are others.  Each of these systems was built to provide a hierarchical (i.e., broad categories with increasingly narrow subcategories) method of categorizing patents according to the technologies represented in the patent.  This facilitates searching for patents in a particular domain.

More recently, the US Patent and Trademark Office (USPTO) and the European Patent Office (EPO) have agreed on a common classification system called the Cooperative Patent Classification system (CPC). This is based on the IPC but includes more levels of detail.

CPC Hierarchy Example

The following example illustrates the hierarchical structure of the CPC:

  • B60H: Arrangements or Adaptations of Heating, Cooling Ventilating, or Other Air-Treating Devices Especially for Passenger or Goods Spaces of Vehicles
  • B60J: Windows, Windscreens, Non-fixed Roofs, Doors, or Similar Devices for Vehicles;...
    • B60J 1/00: Windows; Windscreens: Accessories therefor
    • B60J 3/00: Antiglare equipment associated with windows or windscreens
    • B60J 5/00: Doors
    • B60J 7/00: Non-fixed roofs; Roofs with movable panels
      • B60J 7/02: of sliding type
      • B60J 7/08: of non-sliding type, i.e. movable or removable roofs or panels
      • etc
    • B60J 9/00: Devices not provided for in one of main groups B60J 1/00-B60J 7/00
    • etc
  • B60K: Arrangement or Mounting of Propulsion Units or of Transmissions in Vehicles;...

You can choose to use CPC codes at whatever level of specificity you desire.  For example, if you are looking for patents on removable vehicle roofs, CPC scheme B60J includes non-fixed roofs for vehicles, among other things, but B60J 7/00 is the sub-class that includes only non-fixed roofs, so searching that subclass will save time weeding through other irrelevant items that are part of the broader class, such as windows and doors. 

Likewise, B60J 7/12 is even more specific, including foldable roofs, and B60J 7/1226 includes soft tops for convertible vehicles.



Using the CPC on the USPTO Site

The USPTO website provides a handy tool for finding CPC codes for your topic.  To use this tool, go to  You can see a detailed description of any classification code by typing it in the search box on the left of the web page:  

If you do not have a code to look up, you can use the "Classification Text Search" on the right side of the page:


Make sure the radio button for the desired classification system is selected (1).  Then type the keywords that you brainstormed into the search box (2) and select "search" (3). Browse the search results for the classification(s) that best fit your application and select the link for that scheme:


When one of the "CPC Schemes" is selected, you will then see the hierarchical classification structure:


Using the CPC in Espacenet

Espacenet provides a way to search the CPC system.  To do this, select "Classification search" from the top menu on the Espacenet home page: 


From this page you can search for classifications that contain your keywords (1), or browse the CPC by main category using the first letter of the category (2) or the title and description (3).

You can drill down to more specific classifications by clicking on each classification's (or subclassification's) title.  Once you have found a suitably narrow class, you can find all patents in that class by selecting the box next to the desired class, then selecting "Find patents."