This may sound like a bizarre Trivial Pursuit question, but it’s actually highly relevant in many patent cases.

Let’s start with “prior art.” In patent-professional shop talk, prior art refers to any publication—such as a patent application, patent, or scientific article—that predates the patent application being examined. Because it existed earlier, prior art can be used to argue that an invention is not new (“novel”) or is obvious in view of what was already known.

So what, then, is “secret prior art”?

Secret prior art is a special category of prior art that was filed at a patent office before the application under examination, but was not published until after that application’s filing date. In other words, it technically came first—but you couldn’t have found it at the time.

This makes secret prior art particularly frustrating: it typically cannot be discovered in a patentability search, yet it is often available to—and cited by—patent examiners.

How Is Secret Prior Art Treated? The answer depends on the jurisdiction.

United States: Certain types of secret prior art may be discounted, for example where both applications were filed by the same applicant, subject to specific statutory conditions.

European Patent Office (EPO): Secret prior art is generally relevant only for novelty, not for obviousness.

Israel: Secret prior art is typically relevant only to the extent that the claims of the earlier application overlap with the claims of the later application.

As you can see, the rules vary—and the details matter.

Important note: This article is intended to highlight general principles only. The treatment of secret prior art can be nuanced and fact-specific, so it’s essential to consult your patent representative for advice tailored to your situation.