U.S. patent law doctrine that was expressed in U.S. Supreme Court Cases as
“Where a device is so far changed in principle from a patented article that it performs the same or a similar function in a substantially different way, but nevertheless falls within the literal words of the claim, the doctrine of equivalents may be used in reverse to restrict the claim and defeat the patentee’s action for infringement.”
Graver Tank & Mfg. Co. v. Linde Air Prods. Co., 339 U.S. 605, 608-09, 70 S.Ct. 854, 856, 94 L.Ed. 1097 (1950) and
“The patentee may bring the defendant within the letter of his claims, but if the latter has so far changed the principle of the device that the claims of the patent, literally construed, have ceased to represent his actual invention, he is as little subject to be adjudged an infringer as one who has violated the letter of a statute has to be convicted, when he has done nothing in conflict with its spirit and intent.”
Boyden Power-Brake Co. v. Westinghouse, 170 U.S. 537, 568, 18 S.Ct. 707, 722, 42 L.Ed. 1136 (1898).
The doctrine is intended to cut back on overly expansive claims. It is relatively rarely applied today. See, Doctrine of Equivalence.