Cite as: 532 U. S. 23 (2001)
Opinion of the Court
Ninth Circuit affirmed the judgment. Sarkisian v. Winn-Proof Corp., 697 F. 2d 1313 (1983). Although the Winn-Proof traffic sign stand (with dual springs close together) did not appear, then, to infringe the literal terms of the patent claims (which called for "spaced apart" springs), the Winn-Proof sign stand was found to infringe the patents under the doctrine of equivalents, which allows a finding of patent infringement even when the accused product does not fall within the literal terms of the claims. Id., at 1321-1322; see generally Warner-Jenkinson Co. v. Hilton Davis Chemical Co., 520 U. S. 17 (1997). In light of this past ruling—a ruling procured at MDI's own insistence—it must be concluded the products here at issue would have been covered by the claims of the expired patents.
The rationale for the rule that the disclosure of a feature in the claims of a utility patent constitutes strong evidence of functionality is well illustrated in this case. The dual-spring design serves the important purpose of keeping the sign upright even in heavy wind conditions; and, as confirmed by the statements in the expired patents, it does so in a unique and useful manner. As the specification of one of the patents recites, prior art "devices, in practice, will topple under the force of a strong wind." U. S. Patent No. 3,662,482, col. 1. The dual-spring design allows sign stands to resist toppling in strong winds. Using a dual-spring design rather than a single spring achieves important operational advantages. For example, the specifications of the patents note that the "use of a pair of springs . . . as opposed to the use of a single spring to support the frame structure prevents canting or twisting of the sign around a vertical axis," and that, if not prevented, twisting "may cause damage to the spring structure and may result in tipping of the device." U. S. Patent No. 3,646,696, col. 3. In the course of patent prosecution, it was said that "[t]he use of a pair of spring connections as opposed to a single spring connection . . . forms an important part of this combination" because it
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