Lesson 4: Searching for Patents

4.1 Purposes of a Patent Search

Many first encounter the task of patent searching during an attempt to patent an invention, but there are other reasons to search for patents. Patent searching experts typically categorize patent searching by these reasons, which inform the strategies and tools used. Each type of search may use a combination of strategies, which will be explained in section 4.2, and databases, explained in 4.3. The different types of patent searches fall into two rough categories: patent searching performed for legal reasons and patent searching performed for informational and design reasons.

Patent searching as legal research

Many formalized patent search strategies assume the searcher has a legal need in finding prior art and determining the legal status of a given technology. Legal patent research typically requires exhaustivity; the end goal is to produce a set of all relevant patent literature and then determine the scope of legal coverage via analysis of the claims, terms of protection, and other details. Below are some of the common patent searches performed to determine legal rights.

Note that patent applicants and others invested in determining the legal status of patents frequently employ professional patent searchers (including patent attorneys and agents) to conduct patent searches on their behalf, as experts can often find patent literature that novices cannot. Patent agents and attorneys can also offer interpretations of what is found. This lesson provides tips for preliminary patent searching only.


The goal of a patentability search is to find as much prior art as possible that might be relevant to a potentially patentable invention. The patent applicant (or patent attorney or agent) then analyzes this prior art to determine if they can create a patent application that will meet the requirements of novelty and non-obviousness. Patentability searches need to be as thorough as possible, retrieving granted patents and patent applications from all possible patent offices. It is therefore important to use patent searching databases that cover all necessary ground. To be truly thorough, the patent applicant will also attempt to locate prior art in non-patent literature using scholarly article databases and other resources. This is sometimes called a state-of-the-art search, although this term is also sometimes used to describe market analyses that may not involve patentability (see below).

Freedom to operate

Freedom to operate searches can help determine whether an inventor can produce and market an invention in a specific territory without fear of infringement. Some inventors may not intend to patent their inventions due to the cost and labor of the process or a desire to keep the technology open-source. Some inventors may initially intend to patent their inventions and, during the course of a patentability search, discover that their invention is fully anticipated by prior art and the likelihood of patenting it is low. Freedom to operate searches take patentability out of the equation and focus instead on avoiding infringement. This may involve asking questions surrounding the status of disqualifying prior art, for instance:

  • "What is the geographic scope of relevant patents, specifically, are there relevant patents in the specific country or region in which I want to market my invention?”
  • “Are the relevant patents still in effect or have they expired?”
  • “Was a patent ever granted for this invention, or does the disqualifying prior art appear only in applications for patents that were never granted?”


Validity/invalidity searches establish whether a patent or one or more of the patent’s claims has been granted in error. The searcher attempts to find prior art that was not acknowledged during the patent process to disqualify the granted patent. An inventor might do this to establish freedom to operate or to respond to an infringement suit brought by the owner of the patent.

Patent searching as informational and design research

Most prescribed patent search strategies are geared toward those interested in the legal rights afforded by patents; some experts have identified this as a result of outreach by the USPTO and other entities most interested in fostering economic growth and suggested that these strategies may not suit all patent searchers’ needs.

Patents are an underutilized and rich information resource, full of technical detail and, through their citations of prior art and classification systems, well-suited to following trends in history, business, and technology. Searching patent literature for informational purposes does not require exhaustivity nor as formalized an approach as a patentability, freedom to operate, or validity/invalidity search might.

Product design

Engineers and others engaged in product design can use patent literature to inform their decision making processes. As patent literature includes new and detailed information not available in popular and scholarly sources, it can help designers identify alternate solutions to problems that patented technology attempts to solve, better understand how a piece of technology works and might be put to a new purpose, and get a sense of where technology is heading in a particular field. At the University of Wisconsin-Madison, where faculty, students, and staff are obligated to disclose new inventions (as detailed in Lesson 3 of the Introduction to Intellectual Property micro-course), having a general knowledge of patents in a discipline can also provide a baseline from which to assess whether you have an “invention” to be disclosed.

Although patent searching during the product design process goes generally unacknowledged as a distinct type of patent search, using a patentability search or freedom to operate search as a model for this type of search may prove unnecessarily cumbersome. Instead, these searches may benefit from improvisation, creativity, and databases with easily-learned, flexible functionality. Searchers might initially borrow strategies from legal-focused searches, but find they can search more efficiently for the information needed without such a structured approach.

Although patent searching during the product design process goes generally unacknowledged as a distinct type of patent search, using a patentability search or freedom to operate search as a model for this type of search may prove unnecessarily cumbersome. Instead, these searches may benefit from improvisation, creativity, and databases with easily-learned, flexible functionality. Searchers might initially borrow strategies from legal-focused searches, but find they can search more efficiently for the information needed without such a structured approach.


Needs for historical patent searching vary widely, but might involve identifying patent records for a known invention or patents invented by a specific individual or owned by a specific company. The information at hand may determine the best database and strategy in performing the search. Some factors that might influence a historical patent search include:

  • Is the patent number inscribed on the invention?
  • Do you know the name of the inventor or rights-holder (including name variants, e.g., was the inventor’s preferred name also their legal name, was the company that owned the rights a division of a larger company?)
  • Do you know where the inventor lived and approximately when the device would have been patented?
  • Do you have the device or a photo of the device?
  • Do you know how the device works, what it was/is used for, and what it is made of?


State-of-the-art and landscape searches can help you understand a specific technology or market. These searches combine patent searching on a broad area of technology with accompanying searches of trade and scholarly journals, as well as industry and market research. Goals of these searches include gaining insight into the developed and as-yet-undeveloped technology in an industry, and identifying competition and market peers. Some alternately use the term state-of-the-art searching to refer to an exhaustive patentability search that incorporates non-patent literature (see above).

4.2 Patent Searching Strategies

Regardless of the type of patent search you intend to perform, you will likely need to use a strategy that combines keyword searching with other types of searches, possibly in multiple patent searching databases. The first part of this section explains some basic components you can use in a search strategy. Searching on a single one of these components can sometimes yield promising initial results, but all patent searches benefit from combining approaches. These components are followed by some suggested overall combined search strategies.


Most patent databases allow for some type of keyword searching, although the scope, abilities, and quirks of these searches vary widely. Keyword searching can be an easy first step in patent searching, but it pays to get to know the keyword searching capabilities of the database used.

For instance, the Google Patents keyword search should seem familiar to anyone who uses Google. Users can simply type words in the search box, and Google Patents will search a huge database of scanned patent documents from most international patent offices (in some cases back to 1791) for records in which all of the words occur anywhere within the document. This typically yields lots of results, many of which may not be relevant.

On the other hand, the quick search in the USPTO PatFT database allows you to search specific or all fields of U.S. granted patents back to 1976, but limits you to two words or phrases at a time (the advanced search for USPTO allows for more flexibility, but requires more familiarity with Boolean operators, nested expressions, and familiarity with USPTO fields).

As discussed in Lesson 3 regarding titles of patents, patent applicants may not use intuitive language, colloquialisms, and brand names in either their titles or descriptions. For keyword searching, instead consider the following:

  • What does the invention do?
  • If it’s an innovation upon existing technology, what’s novel about the approach?
  • Is it a process, or is it a product?
  • What is it made of?
  • How is it used?
  • What are some synonyms for it or other words that describe its nature?

Due to differing terminologies, different database coverages, and other factors, even the most diligent, creative keyword searchers will likely miss important patents relevant to their research unless they follow up with additional strategies.

Inventors and assignees

One productive way to begin a search can be to identify a given researcher with expertise in a type of technology, or to identify the maker of a product that closely resembles what you intend to design or patent, and to search for either the inventor name or applicant, assignee, or owner name in the appropriate field.

For some historical searches, this might be the only step required to identify the desired patent(s). For other searches, this may lead to terminology for keyword searches, cited references to explore, and classification codes to expand your search.

Forward and backward citation mining

Once you have discovered at least one relevant granted patent or patent application, you can expand your search using citations to other patent literature. Most patent searching databases include hyperlinks to patents referenced in the patent application or granted patent (with some caveats; for instance, the USPTO PatFT and AppFT databases only link to other U.S. patents referenced, not foreign patents). Many patent databases, including Lens.org, Google Patents, and Espacenet, also link from patent documents to later patent applications and granted patents that referenced them as prior art.

Classification searching

As explained in Lesson 3, patent examiners assign classification codes to patent documents so that similar inventions are grouped together even if the inventors used different terminology when filing their applications.

Cooperative Patent Classification (CPC) is a single system used by the U.S. Patent and Trademark Office and the European Patent Office (EPO) to classify patents and is compatible with the International Patent Classification system used more broadly. Examiners typically assign multiple CPC codes to a single patent.

Each CPC can be broken down by levels of specificity. For example:

A63F 9/16
Group 9 (Games not otherwise provided for)
Main group 16 (Spinning-top games)

Patent Cooperative Patent Classification example
Cooperative Patent Classification example

Augmenting a keyword search with CPC searching may seem like extra work, but patent examiners make heavy use of these codes in their prior art searching, which means that learning to use CPC codes in the patent search process can better help you navigate the literature on their terms. Understanding and using CPC is therefore essential for patentability, freedom to operate, and validity/invalidity searches, but can also helpfully define the scope of a technology for a state of the art or landscape search, and can also unearth invaluable patent literature for the design process.

One way to get to know the CPC system is to experiment with Espacenet’s Classification Search feature, which allows you to search on keywords within the classification definitions and to navigate through the levels of classification by clicking headings, starting with lettered Sections at the top level. An advantage of viewing CPC codes in this way is that you can see neighboring classifications, which may turn out to be just as relevant or even more relevant for your search.

Combined search strategies

As mentioned above, most prescribed search strategies are conceived with the idea of establishing legality--they emphasize a systematic, consistent approach geared toward comprehensiveness. Patent researchers engaged in less formalized product design, state-of-the-art/landscape, and historical patent searching may find that harvesting these strategies for parts may be more productive than following them strictly.

USPTO 7-step U.S. patent strategy

The USPTO offers a 7-step strategy to identify all prior art in U.S. granted patents and patent applications. This strategy requires developing keywords to search a CPC scheme hosted on the USPTO website, then using relevant CPC codes to run searches in the CPC fields of the USPTO PatFT and AppFT databases (described below), which include U.S. patents and patent applications, respectively. As acknowledged in the official strategy document, this is not a truly comprehensive prior art search in that it locates only U.S. patent information rather than all prior art.

University of Maine’s 6.5-step patent strategy

A patent librarian at the University of Maine produced a variation on the USPTO’s recommended strategy that substitutes the more robust classification search on the EPO’s Espacenet database for the USPTO’s CPC scheme. Although it uses the EPO’s classification search, this is still focused on U.S. patent literature. (A patent searcher could, however, expand the search to international patents and patent applications by checking a box next to the desired classification(s) in the Espacenet classification search.)

Keyword searching first, classification second

5 step patent search strategy diagram
Five step patent search strategy, adapted from Dominic DeMarco, 2010: Mechanical Patent Searching: A Moving Target

Although both of the previous strategies begin with brainstorming keywords, they recommend using these keywords to search classification schemes directly. As classification scheme definitions are developed around consistent terminology, this will only yield good results for searchers who luck into using the same terminology as the creators of the scheme.

A way around this is to use brainstormed keywords to search patent literature directly by running searches in databases with full-text access to lots of patents. This may be particularly helpful for concepts and technologies that are too new or specific to have entire CPC classifications dedicated to them. Once the searcher has found records for a few relevant patents, the CPC codes assigned to them can then be used to expand the search to include granted patents and patent applications in which different terminology is used to address the same functionality.

For instance, you might begin using a keyword search in Google Patents, which tends to return many results from U.S. and international patent but does not have a well-developed classification search. You could then create a list of CPC codes from Google Patents for use in Espacenet’s classification search. Alternately, you might run a keyword search in Lens.org, then use Lens.org’s built-in classification search to expand. For each relevant granted patent or patent application, you could also follow up on inventors, applicants/owners/assignees, and backward and forward citations.

In all cases, the key to patent searching is repeating the process, trying new keywords, new classifications, and combinations and, typically, using a combination of patent searching databases.

4.3 Databases

No single patent searching database does everything well. The databases created by the USPTO, EPO, and other patent offices have the most definitive information for those offices; however, third-party databases offer unique search features, user-friendly interfaces, and even full-text searching on patent literature that the official patent office databases may not. Each patent searching website may visualize the information associated with a patent document differently (and may include updated information on details that may change after issuance, such as assignee), but most include downloadable PDFs of the original patent document.

Below is a list of useful patent-searching databases and a brief summary of their content and capabilities as of 2019. Although there are subscription patent searching databases available, these are all free to the public (with some conditions, in the case of PubEAST and PubWEST).

USPTO website databases

PatFT and AppFT

PatFT and AppFT are the USPTO’s main patent searching databases, PatFT for U.S. granted patents, AppFT for U.S. patent applications. These databases contain definitive, official information for USPTO patent literature. Nearly all patent fields are independently searchable; however, full-text searching is available in PatFT only for post-1976 documents and in AppFT only for post-2001 documents. Patents granted between 1790-1976 can be retrieved by issue date, patent number, or CPC.

Recommended for: detail-oriented searchers who need definitive legal information on U.S. patents, historical patent searchers with patent number or issue date information
Potential pitfalls: search interface has a slightly higher learning curve than others (help menu may be necessary to achieve good results), search results screen only provides limited information, records require additional steps to locate and download the full patent document

Public PAIR

The USPTO’s Public Patent Information Retrieval (PAIR) database contains detailed information and files on U.S. patent application material, including a file wrapper tab with details on examiner searches and correspondences between the USPTO and the applicant. Although not intended for thorough prior art searching, it contains unique, invaluable information for those needing information about a specific patent or patent application.

Recommended for: searchers who need all possible info on a specific patent
Potential pitfalls: searchers can only retrieve records by patent numbers, application numbers, and other identifiers, and not by keyword, applicant, or inventor; USPTO resources are particular about numerical format (e.g., commas must be omitted from patent numbers to search on them in Public PAIR)

Patent Assignment Search

Like Public PAIR, the USPTO Patent Assignment Search is not designed for prior art searching, but can be a valuable next step for those requiring more information, in this case, current assignees of a given patent. Using a patent number or other identifier or the name of a current or past assignee, you can view all of the rights holders of a given patent over time.

Recommended for: searchers who need all possible info on a specific patent, potentially those who may need to license technology for their own inventions and need to know who owns the rights
Potential pitfalls: searchers can only retrieve records with specific information, such as patent number and assignee names; USPTO resources are particular about numerical format (e.g., commas must be omitted from patent numbers to search on them in Public PAIR)

Google Patents

Google Patents includes a large number of U.S. and international patent documents scanned in OCR (optical character recognition) format dating back to 1790. This may be the largest corpus of full-text searchable patent literature available online, and the search functionality is very straightforward and forgiving for anyone familiar with basic keyword searching. Searchers cannot search many specific fields of the patent and will not have many options to refine and sort results; however, searchers will rarely find zero results.

Recommended for: novice patent searchers, patent searchers for whom full-text searching has failed in other databases, historical patent searchers with some vague ideas, but without the necessary information to conduct a search in the PatFT pre-1976 collection
Potential pitfalls: the size of the database and Google’s search algorithm often produce large sets of search results that can be hard to sort through, the OCR scanning on older patents is imperfect and leads to relevant patents being missed and irrelevant patents being included in the search results, filtering and sorting options are limited


Espacenet is the official EPO patent search database and includes patent literature from patent offices throughout the world, including the U.S. Having undergone a significant update in late 2019, it now features a multitude of new filters, displays, and sorting options. Patents in English, French, and German are full-text searchable (including OCR-scanned U.S. patents prior to 1976), and a translation feature is available for patents in 30 languages. Espacenet also includes the most complete, user-friendly CPC classification search available. One particularly helpful feature also available in the Lens.org Patent Search (see below) is that a filter for CPC codes includes the number of times a given CPC occurs in a set of search results. Using this feature, searchers can easily identify the CPC codes strongly associated with patent literature in your search, information that can be used to run subsequent searches on those CPC codes.

Recommended for: all searchers, due to its classification search; searchers seeking an authoritative resource on international patents with a lot of functionality
Potential pitfalls: much of the functionality is brand new and the screen layout and amount of functionality can be slightly overwhelming at first

Lens.org Patent Search

Lens.org is a website that includes both patent and scholarly literature databases created by the non-profit Cambia. The site is intended to encourage innovation, mapping scholarly research to inventions and industry. The patent database, which includes U.S. and international patents, offers an extremely user-friendly interface, including easy-to-use advanced filtering features, and it works well for preliminary prior art searching, as well as searching for design ideas. It also includes data visualizations and the ability to export search results in multiple data formats, which can be helpful for state-of-the-art/landscape searchers. As mentioned in the Espacenet description above, it includes a helpful filter for CPC codes that includes the number of times a CPC occurs in a set of search results.

Recommended for: both novice and expert searchers who want the ease of Google Patents (with less full-text coverage) and the CPC classification browse features of Espacenet (also with less full-text coverage) in a handy package with lots of useful features; ideal in particular for those more interested in creative searching rather than comprehensive searching
Potential pitfalls: not as authoritative as Espacenet or USPTO databases; full-text database not as thorough as Google Patents or Espacenet, thus not as equipped for keyword searching, particularly for historical patents


PubEAST and PubWEST are public-facing versions of patent searching tools used by USPTO patent examiners. They allow searchers to query multiple databases of both U.S. patent documents and international patent documents; accommodate full-text, field-specific, and classification searching; and have many powerful features for expert searchers, including the ability to export and save both search results and entire patent documents in bulk. Unlike the databases above, which are all freely available on the web, PubEAST and PubWEST are only available at USPTO Public Search Facilities and by appointment at designated Patent & Trademark Resource Center libraries (including Steenbock Library at the University of Wisconsin-Madison).

Recommended for: expert patent searchers intent on finding all possible prior art, as these databases provide the opportunity to mirror and anticipate the process that USPTO examiners will use to find prior art
Potential pitfalls: both databases are powerful, but have very steep learning curves and may prove frustrating to all but the most committed patent searchers

4.4 Searching for design and plant patents

As U.S. design patents and plant patents do not map directly to patents in other countries and regions, searchers for these types of patents may require different strategies.

Design patents

U.S. design patents are searchable on the USPTO websites. The USPTO does not classify design patents using the CPC system, but rather continues to classify them under the older U.S. classification system. This system presents a good starting point for searchers. One recommended strategy is as follows:

  1. Locate the appropriate design class code (D1-D99) for the subject matter in the design patent classes listed on the USPTO site.
  2. On the Classification Symbol Lookup screen, choose USPC, type the design class code in the Enter Classification symbol field, and click Submit.
  3. From the page of classification subclasses, click on a relevant subclass number to read a definition of the subclass. NOTE: Subclasses in black and all-caps indicate that they include the subclasses directly beneath them, thus, in class D17 for musical instruments, subclass 10 for WIND INSTRUMENT also includes subclasses 11 for cornet or trumpet, 12 for harmonica, and 13 for element or attached. The more specific you can be in determining subclass, the fewer patents you’ll have to read through.
  4. Click the red outlined P next to the desired subclass. This will return a list of granted U.S. patents assigned this subclass.

U.S. design patents are also available in Google Patents and Lens.org and will be included in search results for keyword searches. Searchers can limit their results to U.S. design patents on Google Patents by selecting “Design” as the Type. Searchers can limit their results to international design rights (including U.S. design patents) by selecting Design Right as the Doc Type. Lens.org also includes U.S. Classification as a filter, which can be used to limit to specific U.S. design patent classification codes.

Searchers can also use the WIPO (World Intellectual Property Office) Global Design Database, which includes U.S. design patents, as well as comparable industrial design rights from other countries.

Plant patents

Like design patents, U.S. plant patents are searchable on the USPTO website. As is not the case with design patents, the USPTO classifies plant patents using both the CPC system and the older U.S. classification system. Using the U.S. classification system may yield better results for plant patent searchers.

  1. Start on this list of subclasses for the U.S. class PLT, which is assigned to all plant patents.
  2. From the page of classification subclasses, click on a relevant subclass number to read a definition of the subclass. NOTE: Subclasses in black and all-caps indicate that they include the subclasses directly beneath them, thus subclass 152 for NUT (INCLUDING ORNAMENTAL VARIETY) also includes subclasses 153 for Pecan, 154 for Walnut, and 155 for Almond. The more specific you can be in determining subclass, the fewer patents you’ll have to read through.
  3. On the subclass explanation screen, click the red outlined P next to the desired subclass. This will return a list of granted U.S. patents assigned this subclass.

As mentioned in Lesson 3, U.S. plant patents often contain color photographs in the Drawing Sheets section in order to best represent the appearance of the plant. These are only available in black and white via the USPTO website and most other patent databases; however, current full-color print versions are made available at Patent & Trademark Resource Centers, including Steenbock Library. In addition, the University of Maryland Library and New York Public Library have scanned and made available online many full-color plant patent images.

Check Your Understanding

You had intended to patent and market your product in the U.S. only, but, while searching for patents in Espacenet, you identified a Japanese patent that anticipates your planned claims in full. What kind of search can you perform to see if marketing in the U.S. is still a legal option?

B: Freedom to operate search

A freedom to operate search would help establish whether patents held by others would prohibit you from marketing an invention in a given region. In this case, you would focus on U.S. patents, paying close attention to whether the relevant patented inventions you find are still within their terms of protection.

In discovering a Japanese patent that anticipates your claims, you have effectively established that patentability in the U.S. is unlikely, as this patent counts as prior art and would likely lead to the USPTO rejecting your application on the basis of novelty. You have effectively already conducted a patentability search. Landscape searches and historical searches are focused on identifying trends in an industry or locating specific historical patents, respectively, and are not primarily concerned with legal rights.