Held: When reviewing a district court’s resolution of subsidiary factual matters made in the course of its construction of a patent claim, the Federal Circuit must apply a “clear error,” not a de novo, standard of review. Pp. 4–16.
(a) Federal Rule of Civil Procedure 52(a)(6) states that a court of appeals “must not . . . set aside” a district court’s “[f]indings of fact” unless they are “clearly erroneous.” It sets out a “clear command,”Anderson v. Bessemer City, 470 U. S. 564, 574, and “does not make exceptions or . . . exclude certain categories of factual findings” from the court of appeals’ obligation, Pullman-Standard v. Swint, 456 U. S. 273, 287. The Rule thus applies to both subsidiary and ultimate facts. Ibid. And the function of an appeals court reviewing the findings of a “ ‘district court sitting without a jury . . . is not to decide factual issues de novo.’” Anderson, supra, at 573. Even if exceptions to the Rule were permissible, there is no convincing ground for creating an exception here. Markman v. Westview Instruments, Inc., 517 U. S. 370, neither created, nor argued for, such an exception. There, the Court held that the ultimate question of claim construction is for the judge, not the jury, id., at 372, but it did not thereby create an exception from the ordinary rule governing appellate review of factual matters. Instead, the Court pointed out that a judge, in construing a patent claim, is engaged in much the same task as the judge would be in construing other written instruments, such as deeds, contracts, or tariffs. Id., at 384, 386, 388, 389. Construction of written instruments often presents a “question solely of law,” at least when the words in those instruments are “used in their ordinary meaning.” Great Northern R. Co. v. Merchants Elevator Co., 259 U. S. 285, 291. But if a written instrument uses “technical words or phrases not commonly understood,” id., at 292, those words may give rise to a factual dispute. If so, extrinsic evidence may help to “establish a usage of trade or locality.” Ibid. And in that circumstance, the “determination of the matter of fact” will “preced[e]” the “function of construction.” Ibid.
The Markman Court also recognized that courts will sometimes have to resolve subsidiary factual disputes in patent construction; Rule 52 requires appellate courts to review such disputes under the “clearly erroneous” standard. Application of this standard is further supported by precedent and by practical considerations. Clear error review is “particularly” important in patent cases because a district court judge who has presided over, and listened to, the entire proceeding has a comparatively greater opportunity to gain the necessary “familiarity with specific scientific problems and principles,” Graver Tank & Mfg. Co. v. Linde Air Products Co., 339 U. S. 605, 610, than an appeals court judge who must read a written transcript or perhaps just those portions referenced by the parties. Pp. 4–8.
(b) Arguments to the contrary are unavailing. Sandoz claims that separating “factual” from “legal” questions may be difficult and, like the Federal Circuit, posits that it is simpler for the appellate court to review the entirety of the district court’s claim construction de novo than to apply two separate standards. But courts of appeals have long been able to separate factual from legal matters, see, e.g., First Options of Chicago, Inc. v. Kaplan, 514 U. S. 938, 947–948, and the Federal Circuit’s efforts to treat factual findings and legal conclusions similarly have brought with them their own complexities. As for Sandoz’s argument that “clear error” review will bring about less uniformity, neither the Circuit nor Sandoz has shown that divergent claim construction stemming from divergent findings of fact on subsidiary matters should occur more than occasionally. Pp. 8–11.
(c) This leaves the question of how the clear error standard should be applied when reviewing subsidiary factfinding in patent claim construction. When the district court reviews only evidence intrinsic to the patent, the judge’s determination is solely a determination of law, and the court of appeals will review that construction de novo. However, where the district court needs to consult extrinsic evidence in order to understand, for example, the background science or the meaning of a term in the relevant art during the relevant time period, and where those subsidiary facts are in dispute, courts will need to make subsidiary factual findings about the extrinsic evidence. The district judge, after deciding the factual dispute, will then interpret the patent claim in light of the facts as he has found them. The ultimate construction of the claim is a legal conclusion that the appellate court can review de novo. But to overturn the judge’s resolution of an underlying factual dispute, the appellate court must find that the judge, in respect to those factual findings, has made a clear error. Pp. 11–14.
(d) Here, for example, the District Court made a factual finding, crediting Teva’s expert’s account, and thereby rejecting Sandoz’s expert’s contrary explanation, about how a skilled artisan would understand the way in which a curve created from chromatogram data reflects molecular weights. Based on that factual finding, the District Court reached the legal conclusion that figure 1 did not undermine Teva’s argument that molecular weight referred to the first method of calculating molecular weight. When the Federal Circuit reviewed the District Court’s decision, it did not accept Teva’s expert’s explanation, and it failed to accept that explanation without finding that the District Court’s contrary determination was “clearly erroneous.” The Federal Circuit erred in failing to review this factual finding only for clear error. Teva asserts that there are two additional instances in which the Federal Circuit rejected the District Court’s factual findings without concluding that they were clearly erroneous; those matters are left for the Federal Circuit to consider on remand. Pp. 14– 16.
723 F. 3d 1363, vacated and remanded.