Patent lawyers, courts, and scholars have spent an enormous amount of time and energy over the last twenty-five years trying to determine the rationale and scope of the doctrine of equivalents, an exception to normal patent infringement rules that allows patent owners to extend the scope of patents beyond their literal bounds in certain circumstances. Judges and scholars in the late 1990s suggested that the doctrine of equivalents exception was swallowing the rule, complained that it "lacks a coherent vision" and labeled it the most controversial doctrine in all of patent law. Two of the three most important Supreme Court patent cases decided between 1981 and 2005 concerned the scope of a limitation on the doctrine of equivalents called prosecution history estoppel. The Festo case in particular whipped the patent bar into an unprecedented frenzy, attracting more amicus briefs than any other Supreme Court patent case up to that date.

The Festo case also created a perfect natural test for the importance of patent rules on case outcomes. Before the Federal Circuit's en banc decision in Festo, courts often took a relatively permissive (that is, patent-friendly) stance towards prosecution history estoppel by allowing patent owners to capture ground in court they had unnecessarily given up in patent prosecution. In 2000, the Federal Circuit en banc reversed course, creating an absolute bar against applying the doctrine of equivalents to an element in a patent claim where the patentee had narrowed that element in the course of obtaining the patent. Two years later, the Supreme Court reversed course again, taking a middle ground by creating a presumption that such narrowing of a patent in the PTO creates estoppel, but allowing that presumption to be rebutted in various circumstances. To many patent lawyers, the Supreme Court saved patent law from catastrophe by reversing the absolute bar.


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