48:4 How Tangential Does It Have to Be? Making Sense of Festo’s Tangential Limitations Doctrine



In Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co. (Festo II), the Supreme Court addressed the role of prosecution history estoppel as a limitation on the doctrine of equivalents. The Court announced three specific situations where a narrowing amendment related to patentability does not trigger prosecution history estoppel. One such situation occurs when the rationale behind a patentee’s narrowing amendment bears “no more than a tangential relation to the equivalent in question.” Nine years after Festo II, there is still no consistent definition for when a narrowing amendment is tangential.

This Comment provides the necessary framework to discern whether a tangential limitation triggers prosecution history estoppel. A narrowing amendment contains a tangential limitation when two conditions are met: (1) the patentee argues during prosecution that it made the narrowing amendment solely to distinguish its invention from a feature found in prior art; and (2) the purpose behind the narrowing amendment was to clarify a feature different from the one the patentee is asserting has been infringed under the doctrine of equivalents. Additionally, this Comment argues a limitation found in a narrowing amendment is never tangential if it specifies a limited physical range, so the patentee cannot argue something falling outside that limited range infringes under the doctrine of equivalents.