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Evolution of the Doctrine of Equivalents through Case Law

Posted by Isabel Fox | Sep 30, 2021 | 0 Comments

What is the Doctrine of Equivalents?

The doctrine of equivalents is an equitable, common law, judicially created principle intended to prevent an infringer from making minor, insignificant changes to a patented invention and thus avoid liability for infringement. As in the example given in the previous article (“What is the Doctrine of Equivalents, and How Can It Prove my Patent Infringement Claim?”) regarding the nut and bolt versus a rivet, the patent owner would argue that the rivet is equivalent to a nut and bolt, and assuming all other claim limitations are met, that the patent claim is infringed under the doctrine of equivalents. The alleged infringer may argue, among other things, that a rivet is not equivalent to the nut and bolt because it provides a much stronger attachment. Such an issue is a fact issue that a factfinder – whether judge or jury – would decide.

Winans v. Denmead

Most authorities[1] take the position that the doctrine of equivalents originated in 1853 in the United Supreme Court decision of Winans v. Denmead[2]. In Winans' day, railroad “burden rail cars” – cars used to transport heavy materials like coal or grain – were nearly universally simple rectangular box structures mounted on a rail car frame, with or without a roof. The weight of the material being transported placed varying levels of pressure against the sidewalls of the rail car, with the greatest level of pressure at the bottom of the sidewalls and the least pressure at the top of the sidewalls. This often led to failure of the sidewall and resulting spillage of the material being hauled (and perhaps total loss thereof). Such failures were expensive for the railroad companies in terms of damage to the rail car and lost material.

Winans' solution to this problem was to change the shape of the container holding the product. Winans' new container shape was in the form of the frustum of a cone – the largest diameter portion of the cone with the smaller diameter top removed. A frustum is shown below.

A figure from the Winans patent[3] illustrates how the new container shape was used. The inverted conical frustum was mounted to the rail car frame, and a gate or hatch cover was positioned at the bottom enabling easy dumping of the materials therein.

Winans positioned the container so that it extended below the rail car frame, which lowered the center of gravity of the load being carried and overall improved the stability of the car. In his patent, Winans claimed that his container shape distributed the pressure of the load equally on the sidewall from top to bottom and that it enabled nearly twice as much material to be carried as compared to traditional rectangular box-shaped rail cars.

Winans sued Denmead and others when he discovered they were manufacturing and selling an octagonal (8 sided) container rail car with the 8 sides being arranged in a conical circular fashion. The basic configuration of the Denmead container is shown below:[4]

At trial, the judge ruled “That while the patent is good for … a conical body and not to rectilinear bodies, and it being admitted that the defendants' car was entirely rectilinear, that there was no infringement of plaintiff's patent.”[5]         Winans appealed to the U.S. Supreme Court. In a landmark decision in the field of patent law, the Supreme Court reversed the trial court and created the initial formulation of the doctrine of equivalents. In stating the original doctrine, Justice Curtis, writing for the Court in a 5-4 decision, opined: “the patentee, having described his invention, and shown its principles, and claimed it in that form which most perfectly embodies it, is, in contemplation of law, deemed to claim every form in which his invention may be copied, unless he manifests an intention to disclaim some of those forms.”[6]

Testimony at the trial court had shown that Denmead's octagonal design produced the same functional advantages over traditional rail car design as that claimed in the patent, namely: increased capacity, equal distribution of the pressure of the load in all directions across the sides of the container from top to bottom, and lowered center of gravity. In other words, Denmead's octagonal design attained every advantage over the prior art that was found in Winans' patent.

The Court noted also that in practice no perfect circle can be obtained and rhetorically questioned how close an alleged infringer must be to a perfect circle to infringe the claim. The Court, stating the question should have been left to the jury to decide, reversed the decision of the lower court.[7]

Graver Tank & Mfg. Co. v. Linde Air Products

Graver Tank & Mfg. Co. v. Linde Air Products[8] marks the beginning of the modern formulation of the doctrine of equivalents. Variably known as the “triple identity test” or the “function, way, result test,” it is often stated as: “'if two devices do the same work in substantially the same way, and accomplish substantially the same result, they are the same, even though they differ in name, form, or shape' Machine Co. v. Murphy, 97 U. S. 120, 125.,” and thus infringement under the doctrine is established. In other words, the accused infringing product or process was looked at as a whole to see if it met the triple identity test, rather than looking for whether individual claim limitations could be found. For example, if a patent were to issue claiming a lawn mower with certain features, such as a leaf bagger mounted in a particular way to the mower, and another mower appeared on the market also with a leaf bagger but mounted completely differently, infringement could be found under the doctrine of equivalents even though the latter mower lacked the mounting claim limitations because it met the “function, way, result” test of Graver Tank.

The Court discussed the protections of the patent in the face of imitators, stating:

…courts have also recognized that to permit imitation of a patented invention which does not copy every literal detail would be to convert the protection of the patent grant into a hollow and useless thing. Such a limitation would leave room for—indeed encourage—the unscrupulous copyist to make unimportant and insubstantial changes and substitutions in the patent which, though adding nothing, would be enough to take the copied matter outside the claim, and hence outside the reach of law. One who seeks to pirate an invention, like one who seeks to pirate a copyrighted book or play, may be expected to introduce minor variations to conceal and shelter the piracy. Outright and forthright duplication is a dull and very rare type of infringement. To prohibit no other would place the inventor at the mercy of verbalism and would be subordinating substance to form. It would deprive him of the benefit of his invention and would foster concealment rather than disclosure of inventions, which is one of the primary purposes of the patent system.

Discussing how to determine infringement under the doctrine, the Court  explained that equivalency should be measured in light of the patent itself, the prior art, and the circumstances of the alleged infringement. An “important factor,” according to the Court, in determining equivalency “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.” The Court also noted, almost seemingly as an afterthought, that “[w]ithout some explanation or indication that [the Graver Tank flux] was developed by independent research, the trial court could properly infer that the accused flux is the result of imitation rather than experimentation or invention.”

Stated otherwise, to determine whether there is equivalency between a claimed invention and an alleged infringing product, one measures whether the alleged infringing product does the same work in substantially the same way, and accomplishes substantially the same result, such that they are the same, even though they differ in name, form, or shape in the context of the patent, the prior art, and the circumstances of the alleged infringement with particular emphasis on knowledge of persons skilled in the art as to interchangeability. In addition, the lack of evidence of independent invention or experimentation may be used to determine equivalency. Further, the equivalency determination is a factual decision, not to be disturbed on appeal unless it is clearly erroneous.

In this case the patent in question and the alleged infringing product were concerned with welding fluxes.[9] Linde had purchased a patent[10] that claimed fluxes having certain chemical compositions, namely “a combination of alkaline earth metal silicate and calcium fluoride.” The alleged infringing fluxes Graver Tank used contained “silicates of calcium and manganese—the latter not an alkaline earth metal.” In the Court's own words:

The question which thus emerges is whether the substitution of the manganese which is not an alkaline earth metal for the magnesium which is, under the circumstances of this case, and in view of the technology and the prior art, is a change of such substance as to make the doctrine of equivalents inapplicable; or conversely, whether under the circumstances the change was so insubstantial that the trial court's invocation of the doctrine of equivalents was justified.[11]

The Court further noted that:

"[t]he trial judge found on the evidence before him that the [Graver Tank] flux and the composition of the patent in suit are substantially identical in operation and in result. He found also that [Graver Tank flux] is in all respects equivalent to [the patented product] for welding purposes. And he concluded that “for all practical purposes, manganese silicate can be efficiently and effectually substituted for calcium and magnesium silicates as the major constituent of the welding composition.”[12]

Applying the test for infringement pursuant to the doctrine of equivalents set forth above, the Court found that the triple identity test was met by the trial court. The Court further noted that those skilled in the art and the prior art recognized the interchangeability of manganese, which is not an alkaline earth metal, for magnesium in welding compositions. Concluding, the Court stated: “[i]t is difficult to conceive of a case more appropriate for application of the doctrine of equivalents” and affirmed the lower court decision.

Warner-Jenkinson Co. Inc. v. Hilton Davis Chemical Co.

Warner-Jenkinson Co. Inc. v. Hilton Davis Chemical Co.[13] marked a revisit by the Supreme Court to the doctrine of equivalents formulation of Graver Tank, decided nearly 50 years earlier.

Factually, Hilton Davis Chemical Co. (Hilton Davis) produced dyes. It developed and patented[14] a process for dye production using ultrafiltration to purify dyes at certain pressures and pH levels, amongst other requirements. During prosecution of the patent application, an amendment was filed that amended the claims to require a pH level between 6.0 and 9.0. The upper pH range was to avoid a prior art patent, though why the lower range of 6.0 was chosen was never explained.[15]

After learning that defendant Warner Jenkinson Co. was using a process at a pH of 5.0 to produce its dyes, Hilton Davis sued for infringement under the doctrine of equivalents.

Warner Jenkinson Co made numerous arguments in its defense, among them that the doctrine of equivalents had not survived certain amendments Congress made in enacting the 1952 Patent Act and that Graver Tank should be overruled.

The Court refused to overrule Graver Tank, though in its opinion it did agree that the doctrine of equivalents had, through the years since that opinion was handed down, been stretched beyond its original intent and broadened claim coverage farther than was appropriate. In its discussion, the Court announced what has become known as the “all elements test”, stating:

Each element contained in a patent claim is deemed material to defining the scope of the patented invention, and thus the doctrine of equivalents must be applied to individual elements of the claim, not to the invention as a whole. It is important to ensure that the application of the doctrine, even as to an individual element, is not allowed such broad play as to effectively eliminate that element in its entirety. (Emphasis added.)

The Court thus moved the doctrine of equivalents from looking at an alleged infringing product or process as a whole under the triple identity test to focusing on the individual claim limitations. Under Warner-Jenkinson, then, to show infringement under the doctrine of equivalents, a patent owner must show that an accused product or process practices each claim limitation or its equivalent. This is the state of the doctrine of equivalents today. Note that to show equivalency to a claim limitation, the triple identity (or function, way, result) test may still be used, though the Court declined to rule that that particular test was the only one to be used, letting future court decisions define the measure of equivalency needed and how to determine that measure.

The Court further held that: “whether an accused element is equivalent to a claimed element, the proper time for evaluating equivalency—and thus knowledge of interchangeability between elements—is at the time of infringement, not at the time the patent was issued.”

Warner-Jenkinson raised many other issues, including that Hilton Davis was barred from claiming infringement under the doctrine of equivalents due to file or prosecution history estoppel, which will be discussed in a future article.


The Doctrine of Equivalents provides patent owners with another route to protect their Intellectual Property, either in Federal Court patent infringement actions or in International Trade Commission exclusion procedures. Whether a patent owner or someone accused of infringement, our attorneys can help you by investigating whether patent rights are being infringed either directly or pursuant to the doctrine of equivalents. In either situation you are encouraged to seek legal advice promptly. Please call us at 651-256-9480 or fill out this Contact Form to speak with us before it's too late.

*The information provided in this article is not legal advice and should not be relied on as such. The content of this article is for informational purposes only and is meant as a starting point in your search for answers to your legal questions.

[1] Others challenge this. See, e.g., The Judicial Doctrine of equivalents, 17 Harvard Journal of Law & Technology 247, 254-254.

[2] 56 U.S. (15 How.) 330 (1853).

[3] US Patent No. 5175, issued June 26, 1847.

[4] Copyright owner:

[5] 56 U.S. (15 How.) at 340.

[6] Id. at 343.

[7] Id. at 343-344.

[8] 339 U.S. 605 (1950)

[9] “Weld flux is a welding agent that prevents the weld from interacting with the surrounding medium (like air). The reason why it is so important is that the base and filler material can interact with the atmosphere and cause the formation of oxides or other unwanted compounds. … During a welding process, the base metal and the filler undergo significant temperature changes in a very short amount of time. The heated metal may interact with the surrounding air and cause oxidation, which creates an oxide layer on the weld, reducing the weld strength. And, it is not just oxygen that can create infective welds, the formation of sulfides and nitrides can also hurt the weld's strength.”

[10] U.S. Patent No. No. 2,043,960.

[11] 339 U.S. at 610.

[12] Id. at 611-612.

[13] 520 U.S. 17 (1997).

[14] U.S. Patent No. 4,560,746.

[15] In footnote 2 of the Court's opinion, it was noted that: Respondent counters that the process was successfully tested to pH levels as low as 2.2 with no effect on the process because of foaming, but offers no particular explanation as to why the lower level of 6.0 pH was selected.”

About the Author

Isabel Fox

Isabel Fox is a registered patent agent and has been with Gallium Law since 2018. Her practice largely revolves around utility and design patent prosecution; including conducting patent landscape searches, drafting patent applications, and responding to Office Actions issued by the USPTO...


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