The claims of a patent are central to virtually every aspect of patent law. The claims define the scope of the invention, and their meaning therefore determines both whether a defendant's product infringes a patent and whether the patent is valid. One of the most significant aspects of patent litigation is "claim construction," the process of defining the words of the claim in other, theoretically clearer words. Courts construe the claims of the patent by starting with the plain meaning of their terms as they would be understood by a person having ordinary skill in the art, or PHOSITA. Claim construction occurs in every patent case during a "Markman hearing." Indeed, claim construction is so important to patent litigation that once the court construes the claims, most patent cases settle, and those that do not are often decided on summary judgment. As Judge Rich succinctly put it, "the name of the game is the claim." 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. Indeed, the risk of change in the meaning of terms over time is particularly great in patent law, because patents necessarily involve new ideas, and the process of assigning terms to describe those new ideas is not static. As the Supreme Court recognized many years ago, "it does not follow that when a newly invented or discovered thing is called by some familiar word, which comes nearest to expressing the new idea, that the thing so styled is really the thing formerly meant by the familiar word." In that case, the Court found that the meaning of the term "bridge" in a 1790 statute did not mean the same thing in 1860 after the development of railroad bridges. The term was the same, but its scope had changed over time in response to changes in technology.
Mark A. Lemley,
The Changing Meaning of Patent Claim Terms,
Mich. L. Rev.
Available at: http://repository.law.umich.edu/mlr/vol104/iss1/3