Graver Tank v. Linde: The Doctrine of Equivalents and the Function-Way-Result Test

Graver Tank & Manufacturing Co. v. Linde Air Products Co., 339 U.S. 605 (1950), decided May 29, 1950, is the Supreme Court’s foundational statement of the doctrine of equivalents. It answered a question that has shadowed patent law since its beginning: whether an infringer can escape liability by making an insubstantial change to a claimed invention, substituting one ingredient for a close cousin, and thereby stepping just outside the literal words of the claim. The Court, in an opinion by Justice Jackson, said no. A patent protects more than the letter of its claims. It reaches devices and processes that are equivalent in substance, that do the same work in the same way to reach the same result.

The decision matters because literal claim language can never anticipate every trivial variation an imitator might devise. Without a doctrine of equivalents, the Court warned, a patent would be a hollow and useless thing, and the inventor’s protection would be reduced to worthless verbal formalism. Graver Tank remains the doctrine’s origin point, cited whenever non-literal infringement is at issue.

At a glance

• Case: Graver Tank & Manufacturing Co. v. Linde Air Products Co., 339 U.S. 605 (1950)
• Decided: May 29, 1950; opinion by Justice Jackson; affirmed the finding of infringement under the doctrine of equivalents
• Holding: A product that does not literally satisfy a claim still infringes if it performs substantially the same function, in substantially the same way, to achieve substantially the same result, and known interchangeability of the substituted element supports equivalence.
• Significance: The Supreme Court’s foundational articulation of the doctrine of equivalents and the function-way-result test, the starting point for all non-literal infringement analysis.

The chemistry of the dispute

Linde Air Products owned a patent on a welding composition, a flux used in electric welding. The claims at issue called for a combination of “alkaline earth metal silicate” and calcium fluoride. The patentee’s commercial flux (Unionmelt Grade 20) used calcium and magnesium silicates, and magnesium is an alkaline earth metal, so its silicate fell squarely within the claim.

The accused product, Graver Tank’s flux (Lincolnweld 660), used calcium and manganese silicates, substituting manganese silicate for the magnesium silicate. Manganese is not an alkaline earth metal, so the accused flux did not literally satisfy the claim language. The chemical substitution was real. The question was whether it was substantial.

The trial court found it was not. The evidence showed that manganese and magnesium silicates were known in the art to be interchangeable for this purpose, that the two fluxes worked in the same manner, and that a skilled chemist would have understood the substitution to be a routine one. On that record the trial court found infringement under the doctrine of equivalents, and the Supreme Court affirmed.

The function-way-result test

Justice Jackson’s opinion framed the inquiry in the language that has defined the doctrine ever since. The essence of the doctrine is that one may not practice a fraud on a patent. The Court traced the doctrine’s origin to the nineteenth-century decision in Winans v. Denmead, 56 U.S. (15 How.) 330 (1854), and took the operative test from a later case, quoting Sanitary Refrigerator Co. v. Winters, 280 U.S. 30, 42 (1929): a thing is the same as another for these purposes “if it performs substantially the same function in substantially the same way to obtain the same result.”

Equivalence, the Court emphasized, is not a formula applied in a vacuum. It is determined against the context of the patent, the prior art, and the particular circumstances of the case. An important evidentiary factor is whether persons reasonably skilled in the art would have known of the interchangeability of an ingredient not contained in the patent with one that was. Here the record showed exactly that: skilled welders and chemists knew manganese silicate could stand in for magnesium silicate. That known interchangeability was strong evidence that the substitution was insubstantial.

The Court was careful about what equivalence is not. It is not a question of literal identity, and it does not require that the two things be identical in every respect. Two devices may differ in name, form, or shape and still be the same in the sense that matters to patent law, if they do the same work in substantially the same way and accomplish substantially the same result.

The theory: substance over words

Graver Tank rests on a policy judgment about the limits of language. Claims are drafted in advance, in words, by human beings who cannot foresee every future embodiment. If infringement turned solely on the literal text, an imitator could appropriate the substance of an invention while avoiding its precise verbal boundary, and the patent right would collapse into a drafting exercise. The doctrine of equivalents exists to prevent that outcome, to make sure the protection an inventor receives is coextensive with the invention rather than with the particular words used to describe it.

At the same time, the doctrine is in tension with the public-notice function of claims. Claims tell the world what is and is not protected, and an expansive doctrine of equivalents blurs that line. Justice Black, dissenting, pressed exactly this concern, arguing that the majority had effectively rewritten the claim to cover something the patentee did not claim. That tension between fair protection for the inventor and clear notice to the public runs through every doctrine-of-equivalents case decided since.

The lineage: from Graver Tank to Warner-Jenkinson and Festo

Graver Tank is the doctrine’s headwaters, but the modern law flows through two later Supreme Court decisions that both reaffirmed and disciplined it. In Warner-Jenkinson Co. v. Hilton Davis Chemical Co., 520 U.S. 17 (1997), the Court unanimously held that the doctrine of equivalents survived the 1952 Patent Act and remained good law, while insisting that it be applied element by element rather than to the invention as a whole. That all-elements rule keeps the doctrine from swallowing the claims: each limitation must have an equivalent in the accused device.

In Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., 535 U.S. 722 (2002), the Court addressed prosecution history estoppel, holding that a narrowing amendment made to secure a patent presumptively surrenders the territory between the original and amended claim, limiting the equivalents a patentee can later claim. Together, Warner-Jenkinson and Festo are the guardrails; Graver Tank is the road they guard.

Open questions

The doctrine Graver Tank launched remains among the most unsettled in patent law. The function-way-result test works cleanly for mechanical and chemical substitutions but strains in fields like software and biotechnology, where “way” and “result” are harder to isolate. Courts continue to debate how known interchangeability at the time of infringement, versus at the time of the patent, bears on equivalence. And the interaction between the doctrine and the all-elements rule, especially the “vitiation” concern that finding equivalence would read a limitation out of the claim entirely, generates persistent disagreement. Graver Tank supplied the concept; its boundaries are still being drawn.

Implications for inventors and businesses

• Literal avoidance is not automatic freedom to operate. Designing around a claim by substituting a known-equivalent element may still infringe. A clearance analysis has to consider equivalents, not just the literal words.
• Known interchangeability cuts both ways. For patentees, evidence that the art treated two elements as interchangeable strengthens an equivalents case. For accused infringers, showing that a substitution was a genuine technical advance, not a routine swap, undercuts equivalence.
• Prosecution decisions echo for years. Under Festo, narrowing amendments can forfeit equivalents. Every response to a rejection should be weighed for its long-term effect on the doctrine of equivalents.
• Claim breadth still matters most. The doctrine is a safety net, not a substitute for careful claiming. Broad, well-supported literal claims remain the primary protection; equivalents fill gaps the drafter could not foresee.

Frequently asked questions

What is the doctrine of equivalents? It is a rule that an accused product or process can infringe a patent even if it does not fall within the literal words of a claim, when it performs substantially the same function in substantially the same way to achieve substantially the same result. Graver Tank is its foundational Supreme Court statement.

What was the function-way-result test in Graver Tank? The Court asked whether the accused welding flux performed substantially the same function (welding), in substantially the same way (a silicate flux), to reach substantially the same result. Manganese silicate did so relative to the claimed magnesium silicate, and known interchangeability of the two supported a finding of equivalence.

Is the doctrine of equivalents still good law? Yes. The Supreme Court reaffirmed it in Warner-Jenkinson v. Hilton Davis in 1997 and again addressed its limits in Festo in 2002. Graver Tank remains the starting point, though prosecution history estoppel and the all-elements rule now constrain how far it reaches.

Authorities and sources

• Graver Tank & Manufacturing Co. v. Linde Air Products Co., 339 U.S. 605 (1950). Opinion via Cornell LII.
• Warner-Jenkinson Co. v. Hilton Davis Chemical Co., 520 U.S. 17 (1997), reaffirming the doctrine and adopting the all-elements rule.
• Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., 535 U.S. 722 (2002), on prosecution history estoppel.
• Wikipedia, “Graver Tank & Manufacturing Co. v. Linde Air Products Co.”, for procedural background and holding.
• Bloomberg Law, “Doctrine of Equivalents Is Strong Patentee Tool, but Needs Care”, on modern application of the function-way-result test.