triple identity test

The triple identity test (function, way, result) is a means of determining equivalence in patent infringement claims under the doctrine of equivalents. Courts apply the doctrine of equivalents when the alleged product or process does not infringe on every single exact element in a patent claim, but may nevertheless contain equivalent elements. Under this doctrine, infringement may be found if the difference between a claimed element and the corresponding element in the offending product or process are insubstantial. Equivalence is assessed on an element-by-element basis.

The triple identity test examines:

The function served by a particular claim element,
The way that element serves the function, and
The result thus obtained by that element.

The U.S Supreme Court articulated the triple identity test in Graver Tank & Manufacturing Co. v. Linde Air Products Co., 339 U.S. 605 (1950). In that case, the Court considered whether a substitute ingredient in a welding process was equivalent to an ingredient specified in the patent. The Court explained that equivalence may be found when the accused element performs substantially the same function, in substantially the same way, to obtain substantially the same result as the claimed element. The Court also considered other evidence relevant to equivalence, including whether individuals skilled in the art would have recognized the two elements as interchangeable.

The Supreme Court later addressed the proper role of the triple identity test in Warner-Jenkinson Co., Inc. v. Hilton Davis Chemical Co., 520 U.S. 17 (1997) alongside the “insubstantial differences” test. The Court noted that “the triple identity test may be suitable for analyzing mechanical devices” but often provides a poor foundation for analyzing other products or processes. The Court declined to require courts to use either the triple identity test or an “insubstantial differences” test in every case of alleged infringement. Instead, the Court determined that the relevant inquiry is whether the accused product or process contains elements identical or equivalent to each claimed element of the patented invention. The Court noted that different tests may be more suitable to different cases and emphasized that equivalence must be assessed on an element-by-element basis.

[Last reviewed in August of 2026 by the Wex Definitions Team]

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