We clarify that the time when the narrowing amendment was made, and not when the application was filed, is the relevant time for evaluating unforeseeability, for that is when the patentee presumptively surrendered the subject matter in question and it is at that time that foreseeability is relevant. See Pioneer Magnetics, Inc. v. Micro Linear Corp., 330 F.3d 1352, 1357 (Fed. Cir. 2003). On remand, we asked the parties to brief the following issues:

Prosecution history estoppel serves to limit the doctrine of equivalents by denying equivalents to a claim limitation whose scope was narrowed during prosecution for reasons related to patentability. Pioneer Magnetics, Inc. v. Micro Linear Corp., 330 F.3d 1352, 1356 (Fed. Cir. 2003). Under the rule of prosecution history estoppel, something an inventor gave up during a patent's prosecution in order to obtain a patent cannot be recaptured by the doctrine of equivalents.

[T]he Warner-Jenkinson and Festo presumptions operate together in the following manner: The first question in a prosecution history estoppel inquiry is whether an amendment filed in the Patent and Trademark Office ("PTO") has narrowed the literal scope of a claim. Pioneer Magnetics, Inc. v. Micro Linear Corp., 330 F.3d 1352, 1356 (Fed. Cir. 2003). If the amendment was not narrowing, then prosecution history estoppel does not apply.

First, estoppel applies only if the court determines that "an amendment filed in the Patent and Trademark Office ("PTO") has narrowed the literal scope of a claim." Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co. ("Festo IX"), 344 F.3d 1359, 1366 (Fed. Cir. 2003) (citing Festo VIII, 535 U.S. at 740; Pioneer Magnetics, Inc. v. Micro Linear Corp., 330 F.3d 1352, 1356 (Fed Cir. 2003)). This first step requires us to identify the relevant amendments in the '777 application.

An amendment narrows the literal scope of a claim when it renders a general description more specific, as when a genus is replaced with a species. See, e.g., Pioneer Magnetics, Inc. v. Micro Linear Corp., 330 F.3d 1352, 1356 (Fed. Cir. 2003) ("[T]he amendment of claim 1 from a 'multiplier' to a 'switching analog' multiplier narrowed the literal scope of the claim."). Here, it is plain that Duramed's amendment narrowed the literal scope of the claim.

The patentee may overcome this presumption by demonstrating that the reason for the amendment is unrelated to patentability. Pioneer Magnetics, Inc. v. Micro Linear Corp., 330 F.3d 1352, 1356 (Fed. Cir. 2003). Prosecution history estoppel, including "the determinations concerning whether the presumption of surrender has arisen and whether it has been rebutted, [is a] question[] of law for the court, not a jury, to decide."

a. The Court's Original Analysis: Anomalous as it may seem, the jury plays no role in any of this. Although it is clear that the equitable doctrine of prosecution history estoppel is for the court to decide, see, e.g., Pioneer Magnetics, Inc. v. Micro Linear Corp., 330 F.3d 1352, 1356 (Fed. Cir. 2003); CAE Screenplates Inc. v. Heinrich Fiedler GmbH Co. KG, 224 F.3d 1308, 1319 (Fed. Cir. 2000), the Supreme Court in Festo II did not provide guidance as to whether the determination that the patentee has rebutted the "presumption" is a question for the court or the jury. Though the bench trial in this case moots the issue here, the question generally remains.

At the first step, the inquiry is whether one or more amendments filed in the Patent and Trademark Office ("the PTO") have narrowed the literal scope of the claim. Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., 344 F.3d 1359, 1366 (Fed. Cir. 2003) (" Festo II") (citing Pioneer Magnetics, Inc. v. Micro Linear Corp., 330 F.3d 1352, 1356 (Fed. Cir. 2003) ). If the amendment did not narrow the claim, the analysis ends and prosecution history estoppel does not apply.

The first question in a prosecution history estoppel inquiry is whether an amendment filed in the Patent and Trademark Office ("PTO") has narrowed the literal scope of a claim. Pioneer Magnetics, Inc. v. Micro Linear Corp., 330 F.3d 1352, 1356 (Fed. Cir. 2003). If the amendment was not narrowing, then prosecution history estoppel does not apply. But if the accused infringer establishes that the amendment was a narrowing one, then the second question is whether the reason for that amendment was a substantial one relating to patentability.

Prosecution history estoppel limits the doctrine of equivalents "by denying equivalents to a claim limitation whose scope was narrowed during prosecution for reasons related to patentability." Pioneer Magnetics, Inc. v. Micro Linear Corp., 330 F.3d 1352, 1356 (Fed. Cir. 2003). Prosecution history estoppel was established "to hold the inventor to the representations made during the application process and to inferences that may reasonably be drawn from the amendment.