The Changing Meaning of Patent Claim Terms

30 Pages Posted: 7 Mar 2005


In order to construe the claims of a patent, the court must fix the meaning of the claim terms as of a particular point in time. Both the knowledge of the PHOSITA in a particular field and the meaning of particular terms to that PHOSITA will frequently change over time. But at which point in time shall we fix the meaning of the claims?

It is a fundamental principle of patent law that the time at which we determine the meaning of claim terms varies depending on what legal rule is at issue. Where the question is one of novelty or nonobviousness - whether the invention is truly new - the courts compare the patented invention to the prior art as both were understood at the time of the invention. Where the question is one of enablement or written description - whether the inventor understood and described the invention in sufficient detail - courts evaluate the adequacy of the disclosure based on the meaning of the claims at the time the patent application was filed. Where the question involves the meaning of a special patent claim element called a means-plus-function claim, courts evaluate the scope of that claim element at the time the patent issues. And where the question involves alleged infringement of the patent, courts evaluate infringement in at least some circumstances based on the meaning of the claim at the time of infringement.

An equally fundamental principle of patent law is that patent claims must be construed as an integrated whole. In particular, patentees (or accused infringers, for that matter) are not permitted to argue that a patent claim means one thing when it comes to validity and something else entirely when it comes to infringement. Instead, courts give claims a single meaning in any given case, engaging in only one act of claim construction for any given patent.

These two principles contradict each other. In this paper, I seek to resolve this conflict. In Part I of this paper, I document the distinguished pedigree of both principles. In Part II, I argue that patent claim terms should have a fixed meaning throughout time, and that that meaning should be fixed at the time the patent application is first filed. Part II also discusses some complications that arise as a result of the prosecution process, and how to deal with the problem of later-developed technology.

Suggested Citation

Lemley, Mark A., The Changing Meaning of Patent Claim Terms. Michigan Law Review, Vol. 104, p. 101, 2005; Stanford Public Law Working Paper No. 107. Available at SSRN: or

Mark A. Lemley (Contact Author)

Stanford Law School ( email )

559 Nathan Abbott Way
Stanford, CA 94305-8610
United States

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