How Prior Art Searching Works

What counts as prior art is set by section 102, and it is broader than patents. Under section 102(a)(1) it includes anything showing that "the claimed invention was patented, described in a printed publication, or in public use, on sale, or otherwise available to the public before the effective filing date of the claimed invention". The categories are explained on what counts as prior art. This page is about the practical part: how anyone goes looking for it.

Patent examiners do this as part of examination. MPEP 2141 tells examiners that once a claim's scope is set, they "determine what to search for and where to search", and that "The search should cover the claimed subject matter and should also cover the disclosed features which might reasonably be expected to be claimed." The methods below are the ones open to anyone with a browser.

Four Ways In

Each of the four reaches some documents and misses others, so a search that uses several of them reaches more than one that uses only one. Each is described below.

Classification: The Cooperative Patent Classification

The Cooperative Patent Classification, or CPC, is described on its own site as "a common, internationally compatible classification system for technical documents, in particular patent publications", and as the result of a partnership between the European Patent Office and the USPTO. It is a hierarchy of codes. A short path through it shows the shape:

The point of a code is that it groups documents by subject rather than by vocabulary. Two patents that describe the same mechanism in different words can sit under the same symbol, so a search by symbol reaches documents a keyword search would pass over. Patent pages on Google Patents list the CPC codes assigned to each document, so the codes on one relevant document name groups where others like it may sit.

The limit is built into what the scheme is for. Its site says it will be used by both offices "in the patent granting process". A paper, a README, a conference talk or a blog post did not go through that process, so a classification search over patent documents does not reach it. WIPO's PATENTSCOPE page lists the International Patent Classification (IPC) among its search criteria; the CPC site says the CPC was built to comply with IPC standards.

Keywords, and Why Words Are the Weak Point

A keyword search matches strings. The test a document is measured against does not. MPEP 2131 states that the elements of a claim "must be arranged as required by the claim, but this is not an ipsissimis verbis test", meaning identical wording is not required. A document can describe the same arrangement in entirely different terms and still be the reference that matters. A search that matches only one vocabulary is matching on the property that test sets aside.

Searchers widen keyword searches in familiar ways: synonyms, broader and narrower terms, the function a part performs rather than its name, and older terms for the same idea. Search tools support this with operators. The USPTO's Patent Public Search page links its own references on operators, searchable indexes and stopwords. WIPO's PATENTSCOPE page describes search by keywords, IPC codes, chemical compounds and numbers "in different languages". That matters because a document in another language is as invisible to an English keyword as one in other words.

Citations: Backward and Forward

Patent documents point at one another. Backward citations are the documents a patent cites. Forward citations are the later documents that cite it. Following both from one relevant document is citation chasing, and it reaches documents that neither classification nor keywords surfaced.

On a Google Patents page these appear under the headings Patent Citations, Non-Patent Citations, Cited By, and Families Citing this family, with each entry marked as cited by the examiner or cited by a third party. The examiner's citations are the trace of a search someone already ran: the MPEP refers to an Office policy "requiring recordation of the field of search and search results". The Common Citation Document goes a step further. The EPO describes it as a single point of access to citation data for the patent applications of the five largest IP offices (IP5), which "consolidates the prior art cited by all participating offices for the family members of a patent application".

Citations only connect documents someone chose to connect. A document nobody cited is not reachable this way, and forward citations do not exist until later documents publish.

Non-Patent Literature

Prior art is not only patents. Section 102(a)(1) names printed publications and anything otherwise available to the public. The manual's examples of the second include "a student thesis in a university library" and "a document electronically posted on the Internet" (MPEP 2152), and it records a forum posting that counted as prior art because it was directed to skilled readers and sufficiently disseminated (MPEP 2128). The test behind printed publication is findability, explained on printed publication means findable.

Patent databases are not built for this material. Some help at the edges: PATENTSCOPE carries a non-patent literature collection, and Google Patents lists the non-patent documents a patent cites. For software, much of what was public before a filing date lives in documentation, papers, talks and discussion threads, and reaching it means searching those sources directly. Whether a public code repository is itself a printed publication is a question the manual sections these pages draw on do not answer. That is an open question, not a permission.

The Public Search Tools

Four search tools, run by two patent offices, WIPO and Google, are open to the public. What each covers is stated here from its operator's own page.

The CPC scheme itself is browsable at the CPC site and in the USPTO's classification pages. Citation data across the IP5 offices is at the Common Citation Document.

Why a Search Is a Sample, Not a Census

A search can only return what is in the index it searched. Several kinds of prior art can be missing from every index on the day someone searches.

• Applications not yet published. Under section 102(a)(2), a patent or a published application naming another inventor counts, subject to the exceptions in section 102(b)(2), if it "was effectively filed before the effective filing date of the claimed invention", and it is treated as effectively filed "as of the actual filing date" unless it is entitled to an earlier priority or benefit date (section 102(d)). An application filed before yours that has not yet published is in none of the tools above, and may still count once it publishes. What counts as prior art explains this lane; Lynk Labs v. Samsung shows the same effect under the older, pre-AIA statute.
• Documents no search engine indexes. The printed-publication test asks whether an interested skilled person "exercising reasonable diligence, can locate it" (MPEP 2128, quoting In re Wyer). The manual also quotes the Federal Circuit that "indexing or searchability is unnecessary for a reference to be a printed publication", while recording a separate decision in which a technical report with deficient indexing was held not to be one. Whether a document sits in a search index is not the test, although the manual shows it can bear on the answer.
• Things that are not documents. Section 102(a)(1) also names public use and sale. Those are activities. The manual records the Supreme Court's holding that "an inventor’s sale of an invention to a third party who is obligated to keep the invention confidential can qualify as prior art" (MPEP 2152, quoting Helsinn). No database holds a confidential sale.
• Documents in other words or other languages. Covered above. The search found what its terms matched.

What Finding Nothing Means

Two more points from the manual explain why an empty result answers less than it seems to.

• Anticipation is one test, and obviousness is another. Under MPEP 2131, "A claim is anticipated only if each and every element as set forth in the claim is found, either expressly or inherently described, in a single prior art reference". A search that turns up no single document containing everything has not addressed obviousness, where a disclosure "may be cited for all that it would reasonably have made known to a person of ordinary skill in the art" (MPEP 2152). The difference is set out in novelty and obviousness are different tests.
• Relevant art can sit outside the field. For obviousness, MPEP 2141 reminds examiners that "prior art can be either in the field of the inventor’s endeavor or be reasonably pertinent to the particular problem with which the inventor was concerned". The Federal Circuit applied the second of those routes on 14 August 2026 in Nielsen Co. v. TVision Insights, affirming that a facial-analysis reference was analogous art to a television measurement patent because it had "a same purpose". The decision is recent: as of 13 November 2026 the court's site lists no later order in the case. A search confined to one field can miss art that this rule reaches.

The reverse holds too. A result that looks close is not a finding that anything is anticipated or obvious. Either conclusion is a determination about a specific claim and a specific document, and a search result is not that determination.

Searching an Invention You Have Not Filed

A search query is text sent to a server someone else runs. For an invention that has not been filed, the question worth asking is what that operator records.

On the legal side, the sources these pages draw on say nothing about search queries. Section 102(a)(1) turns on whether the claimed invention was available to the public, and none of the statute or manual sections this page draws on addresses whether a person's own search query discloses anything. This page does not call a query a public disclosure, and it does not call a query safe.

What can be read is each operator's own privacy policy:

• Google. Its privacy policy lists "Terms you search for" among the activity it collects, and describes "processing the terms you search for in order to return results".
• USPTO. Its privacy policy, which the Patent Public Search footer links to, says its web servers save "the date and time, the originating IP address, the object requested, and the completion status of the request" for each request, and that an authorized Google Analytics agent collects comparable information on its behalf.
• WIPO. Its privacy policy says web analytics tools may collect the account username for logged-in users, the IP address, the WIPO websites visited, and the time, duration and date of the visit, and that this information is used in an aggregated and anonymous manner for statistical purposes.
• EPO. Its practices are set out on its data protection and privacy page.

A privacy policy says what an operator collects. It does not settle any legal question about what collection means, and the same question applies, under its own terms, to a general-purpose AI assistant asked to look for prior art. The frameworks that do bear on storing pre-filing material with third parties are covered on patent documents and the cloud, and the reasoning behind keeping pre-filing work on your own machine is on local-first patent discovery.

Where Our Scanner Fits, and Where It Does Not

Our concept scanner is not a prior art search tool. It reads your own code and describes what is distinctive in it. It does not search patent databases, check novelty or assess patentability. That is a different job from everything on this page; see what the tools do and do not do.

What This Page Does Not Decide

• It does not say whether any invention is novel or non-obvious, and it does not walk anyone through reaching either conclusion.
• It does not say whether any search was thorough enough for any purpose.
• It does not say whether a repository, a posting or any other act was a disclosure under section 102.
• It describes the America Invents Act regime. Applications filed before 16 March 2013 can be governed by different rules.

Educational, not legal advice. What a search result means for a particular invention is a question for a registered patent practitioner.