Claims in an issued patent protect a patented invention against infringement.  In a case involving alleged infringement, 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., 117 S. Ct. 1040, 41USPQ2d 1865, 1875 (1977). As to whether an element is an equivalent, “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.”  41 USPQ2d at 1875.  One interpretation of this case law is that an allegedly infringing substitute element could be found an equivalent to a claimed element in one case.  Yet, the exact same substitute element might not be an equivalent to the same claimed element in another claim in another (unrelated) patent.  On the surface, this might appear paradoxical.  Equivalents are claim-specific and case-specific.  It is the context of the claim that determines whether an allegedly infringing element is an equivalent to a claimed element.

Let’s play around with some hypothetical examples.  Suppose a claim recited some components and a wheel that is driven by these components.  Suppose the specification disclosed an automobile wheel as an example of a type of wheel that could be driven as part of the claimed invention.  Perhaps, one could find that a bicycle wheel or a locomotive wheel is an equivalent to the automobile wheel, as any of these could be driven by the claimed components.  Now, suppose a different patent discloses and claims an automobile wheel with specific features and a new type of tire that fits those specific features.  In this case, a locomotive wheel would likely not be found an equivalent, as locomotive wheels don’t have tires.  The locomotive wheel plays a substantially different role from the automobile wheel, in terms of a relationship with a tire.  It is worth keeping in mind the doctrine of equivalents when crafting claims.  Having an awareness of client and competitor product lines, in the context of research and industry trends, helps us frame relevant questions: Can a claim term be broadened to include an equivalent?  Should it?  Does it do so already?  What might an infringer do?  How much detail should be in the specification to support equivalents?  The doctrine of equivalents helps us answer these questions.