In a 7-2 decision authored by Justice Breyer, the U.S. Supreme Court held yesterday that an “appellate court must apply a ‘clear error,’ not de novo, standard of review” to the evidentiary underpinnings of a district court’s claim construction determination. Teva Pharmaceuticals USA, Inc. v. Sandoz Inc., No. 13-854, slip op. at 1-2 (U.S. Jan. 20, 2015). But what are those evidentiary underpinnings? Apparently everyone was in agreement that “when the district court reviews only evidence intrinsic to the patent (the patent claims and specification, along with the patent’s prosecution history), the judge’s determination will amount solely to a determination of law, and the Court of Appeals will review that construction de novo.” Id. at 11-12. In contrast, when a district court relies upon extrinsic evidence to construe the claims, it “will need to make subsidiary factual findings about that extrinsic evidence.” Id. at 12. It is this fact finding that is entitled to deference under Federal Rule of Civil Procedure 52(a)(6). Nevertheless, the Court was clear that “the ultimate question of construction will remain a legal question.” Id. at 13. Therefore, “[f]or example, if a district court resolves a dispute between experts and makes a factual finding that, in general, a certain term of art had a particular meaning to a person of ordinary skill in the art at the time of the invention, the district court must then conduct a legal analysis: whether a skilled artisan would ascribe that same meaning to that term in the context of the specific patent claim under review.” Id. at 12.

Prior to yesterday’s decision, the Federal Circuit had reviewed all of claim construction de novo. Based on the 1996 decision in Markman v. Westview Instruments, Inc., 517 U.S. 370 (1996), claim construction has been treated as a question of law, which under the Federal Rules would not give rise to deference to trial court findings. However, the majority found that courts have been oversimplifying the holding of Markman, stating that the decision stands for the proposition that while “the ultimate issue of the proper construction of a claim should be treated as a question of law, [it] also recognized that in patent construction, subsidiary factfinding is sometimes necessary.” Teva, slip op. at 6. Under Rule 52(a)(6), “a court of appeals ‘must not . . . set aside’ a district court’s ‘[f]indings of fact’ unless they are ‘clearly erroneous.’” Id. at 4 (quoting Fed. R. Civ. P. 52(a)(6)). Because Markman did not create an exception to that rule, it governs claim construction and compels the application of a deferential “clearly erroneous” standard to the findings of fact in claim construction.