Patent Searching Through the US Patent Classification System

During my time as a patent professional, I have been able to gain a fair amount of experience conducting patent searches. Although searching through the USPTO database for a specific disclosure can be more of an art than science, I have come up with a relatively easy method that takes advantage of the USPTO’s patent index system. In this post, I’m going to briefly describe the index that the US Patent and Trademark Office uses to classify all patents. Then I’ll provide my method for using that system to conduct a patent search.

Keep in mind that patent search results are never conclusive. The patent database has over 8 million patents, so even when you find a needle in the haystack, it is impossible to know that there is not another, better (i.e. more relevant) needle in the stack. The process just has some inherent uncertainty built into it.

The USPTO indexing system is called the U.S. Patent Classification (USPC) System. The USPC System is a classification system of U.S. patents organized by subject matter. During examination, each patent application is classified into one or more class/subclass classifications based on the claimed subject matter of the application. When searching for patents related to a specific technology area, one can limit a search to a specific relevant class/subclass, which is a more reliable way to narrow a search than just using keyword searching.

Keyword Searching

The simplest way to conduct a patent search is keyword searching. It involves:

  1. Determining keyword alternatives
  2. Search all keywords and alternatives
  3. Read through results of all searches for matches

There are two main problems with keyword searching:

  • Overinclusive: if another patent even mentions one of your keywords, even if it actually relates to a different subject matter, it will show up as a result and you then must read through it to know if it is relevant or not; and
  • Underinclusive: there could be (and usually are) other ways to describe the target idea that you didn’t think of when determining the keyword alternatives. Patents that you want to see might not show up in the results.

As a result of these two problems, keyword searches result in a much larger haystack that still probably doesn’t include all relevant results. To manage the size of the haystack while (at least, attempt to) including relevant results, we should use the USPC System.

The Index to the USPC System

The USPC System Index is an alphabetical list of the various technological subject matters and their respective class and subclass numbers. The index has up to six levels of indentation, corresponding to up to six levels of subclasses under each class. Each subclass drills down in further detail. Class definitions, lists, and other information on the classification system can be found at That webpage is the main menu for the USPC System.

Searching the USPC Index

The index can be found on the U.S. Patent and Trademark Office’s website at The US Patent and Trademark Office provides the following steps to find a target classification:

  1. Search through the alphabetical Index for the subject matter that best describes the target subject matter. If a match is not found, search through the index for alternatives terms with the same meaning. Search for:
    • broader scope,
    • narrower scope,
    • synonymous terms, or
    • related functions
  2. Identify the relevant class
  3. Search through the first layer of subclasses of the relevant class to identify any related subclasses
  4. Search through the first layer of subclasses within the previously-identified subclass to identify any further related subclasses
  5. Repeat the previous step until there are no more subclasses within the identified class or subclass

The Index to the USPC System includes definitions and examples for classes and subclasses. Viewing these definitions can help guide a classification search.

Search Illustration

Let’s say you want to identify the class (and subclasses) associated with a carabiner used in rock climbing. You first click on the “C” on the index page, and search the “C” page for “carabiner”–but do not locate it there. You then broaden the scope of your search to “rock climbing”–click on the “R” to see that index page. You are unable to find “rock climbing,” so you think of a synonymous/broader term and decide to search “fasteners.” You click “F,” search for that term, and find that the closest category listed is “Separable-fastener, two part,” which links to “24 / 572.1+”–which refers to Class 24 (“Buckles, Buttons, Clasps, etc.”) and subclass 572.1 (“Separable-fastener or required component thereof (e.g., projection and cavity to complete interlock)
“). The plus sign means that additional subclasses within 572.1 are also relevant. By scrolling through the subclasses within subclass 572.1, you can figure out if any subclasses are relevant for your search.

The problems that the above-example illustrates is that correctly coming up with the search term “Separable-fastener, two part” had a low likelihood of success at the outset of the search. The search terms one would most likely use, “carabiner,” “rock climbing,” etc. are not in the index. Thus, my searching method (below) is a more efficient alternative classification search method.

How I Conduct Patent Searches

Instead of searching through the USPC Index, my method involves looking for patents in a similar field and noting that patent’s classification. The USPTO patent database can be accessed here: then click on “quick search” under the heading “PatFT: Patents”. Here are the steps:

  1. Search the USPTO patent database by keyword for the subject matter of interest.
  2. Find a recent patent that covers a similar device, process, composition, etc.
  3. Note the classification number(s) of that patent
  4. View the class and subclass definitions to verify that they cover your invention
    • If not, go back to step 2 and select a different patent

Search Illustration

Let’s try to do the same search for the class/subclasses which include a carabiner. First, you go to the patent keyword search and type in “carabiner.” The results page contains many hits. The first result that looks relevant is Patent No. 8,001,663 for “Safety Carabiner” (your results will vary as newer patents are granted). Click on that patent and look at the information under “Current U.S. Class”: “24/599.5” and “24/600.1” You can view the class descriptions by searching for those classes from the USPC main menu ( typing in the class/subclass numbers, clicking the radio button for “Class Schedule (HTML)”, and clicking “Submit.” You will see a listing of subclasses, including your selected subclass. Clicking on a subclass will open a new window that depicts a hierarchical view of the subclass. A reading of what the subclass is should start at the top of the hierarchy and continuing down each step–each subclass includes and further details the description of its parent subclass. By clicking on the “Definition View,” you can get more information on the meaning of each subclass description to determine whether you’ve found the class/subclass you were looking for.

Once you determine the target class/subclass, it is a trivial matter to open a list of all patents within the class and review each one. Because you used the index, your haystack is smaller and better focused than if you had just done a keyword search.


The USPC System is a useful tool to use when searching for prior patents because it is organized by subject matter. Although navigating the search tools can be tricky, this article provides an easy shortcut to finding your target class/subclass. Once one knows the classes and subclasses her invention falls under, she can conduct a (more) thorough prior art search of the relevant technology classes.