Patent law cases are rare in Washington. Thys v. Rivard, the most recent, concerned patent infringement and turned on two important rules of patent law: the doctrine of equivalents and the doctrine of file wrapper estoppel. Although the former of these rules had confronted the court on an earlier occasion, file wrapper estoppel was presented for the first time by the Rivard case. File wrapper estoppel, a rule of patent construction, derives its name from the Patent Office file wrapper containing the record of proceedings leading up to the grant of the letters patent. If, when the Patent Office examines the application for patent, it is found in the light of the prior art in the field that the applicant has drawn his claims too broadly for his actual invention they will be rejected. Should the rejection be made final, two alternatives are open to the applicant: appeal to the federal courts or amend the claims to meet the grounds of rejection. If the latter course is followed and the claims are narrowed to avoid conflict with the prior art cited against them, the applicant's rights under the resulting patent are limited by file wrapper estoppel. That is, a court construing the patent will not extend its claims to the point where they become equal to or greater than those originally in the application. The theory is that the patentee is estopped from asserting an interpretation of his patent claims so broad as to recapture subject matter once cancelled from the application in order to secure allowance of the patent.
O. M. Christensen,
File Wrapper Estoppel,
23 Wash. L. Rev. & St. B.J.
Available at: https://digitalcommons.law.uw.edu/wlr/vol23/iss3/7