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  • Aishwarya Aher

What is Doctrine of Equivalents?

While there are multiple ways to skin a cat (see discussion on indirect infringement and direct infringement here), patent infringement can occur in two forms. The first is literal infringement. Literal infringement is when every aspect of the claim is being infringed upon in much the same way the claim was written. The second one, more complex, is an infringement under the doctrine of equivalents. Under this doctrine, the patentee can argue infringement even if each and every claim element of the patent is not completely or identically present in the infringed invention. The purpose of the doctrine is to ensure that the infringer does not benefit from minor or insubstantial changes that may escape literal infringement.


The doctrine of equivalents arises in the context of an infringement action. If an accused product or process does not literally infringe a patented invention, the accused product or process may be found to infringe under the doctrine of equivalents. The essential objective inquiry is: "Does the accused product or process contain elements identical or equivalent to each claimed element of the patented invention?" Warner-Jenkinson Co. v. Hilton Davis Chemical Co., 520 U.S. 17, 41 USPQ2d 1865, 1875 (1997). In determining equivalence, "an analysis of the role played by each element in the context of the specific patent claim will thus inform the inquiry as to whether a substitute element matches the function, way, and result of the claimed element, or whether the substitute plays a role substantially different from the claimed element."


The Doctrine of Equivalents was established in the United States with the case of Winans v. Denmead, which dealt with changing a part of the construction of the patented invention to avoid infringement. The patent at issue covered a process for making the body of a sheet-iron railroad car with various specifications. The defendant was alleged to have copied everything other than the physical shape of the car. Setting a precedent, the court held that infringement may be claimed even if the same literal legal patent language was not used. A mere change in form while retaining the rest from the patented claim is still considered infringement. The doctrine is explained in the words of Judge Curtis for the case as, "the patentee, having described his invention, and shown its principles, and claimed it in that form which most perfectly embodies it, is, in contemplation of law, deemed to claim every form in which his invention may be copied, unless he manifests an intention to disclaim some of those forms."


Graver Tank & Mfg. Co. v. Linde Air Prods., Inc. established a triple identity test for equivalence. Linde Air Prods held the patent for an electric welding process. The patent was essentially a composition of the chemical configuration claim for the process. Linde Air Prods brought an infringement lawsuit against Graver Tank for using a chemical replacement with the same properties and resulting in a similar process. The replacement was calcium and magnesium for calcium and manganese. The replacement was considered stealing the benefit of the characteristics of the original composition. To be able to find a substitute, the original composition was used and hence there was an infringement. The court called it an imitation rather than experimentation.


Warner-Jenkinson v. Hilton Davis Chemical Co built the base further. The case is monumental as the Supreme Court recognized that patent rights cannot be secured if anyone from the public is free to imitate a part or parts to it to include in another invention to avoid infringement. The patent needs to be protected as a whole, as well as in parts or elements essential to it. This case established the ‘all elements’ test for equivalence. Under this rule, the doctrine is applied to every individual claim element present in the patent. To validate the test, a product must match the function, method, and result of the claimed elements.


Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co. is a case that helped put limitations on the doctrine and cemented it. The Doctrine of Equivalents is limited by the Doctrine of Prosecution History Estoppel. It is now determined as an important tool to determine the scopes of patent claims. The occurrence of this is recognized when the inventor narrows the claims of the application during the prosecution to avoid or overcome a rejection. In accordance with prosecution history estoppel, claims have to be viewed in light of the rejections, cancellations, and amendments made through the course of the prosecution. The subject matter that gets omitted cannot seek protection under the doctrine of equivalence. However, if the changes made by the inventor to the patent application are not concerning the patent claim, then they will not be limited by the estoppel.


The Doctrine of Equivalents has therefore evolved with time. It began with Winans v. Denmead when the Supreme Court ruled that infringement may be exclaimed even if the literal language is not used. This judgment evolved with Graver Tank & Mfg. Co. v Linde Air Products Co. to what it is today. Infringement may be ruled even if no literal infringement is found but there is an equivalent method or claim in the accused invention. Warner-Jenkinson Co. v. Hilton Davis Chemical Co. came next to help establish the scope of the doctrine of equivalents. It was important to establish that each and every claim in the invention is crucial to the scope of the patented invention and hence the doctrine must be applied to every element individually and not as a whole. The court also decided that the determination of equivalence shall be made at the time of the alleged infringement and not at the time of the patent issuance.


Given those limitations, the doctrine of equivalents continues to be a powerful tool for plaintiffs and their counsel that serves to strength proprietary rights - covering minor changes that may have otherwise helped competitors escape infringement liabilities.

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