(This article is written by Aarushi Vats, Chaudhary Charan Singh University, B.A. LL.B., 5th Year, during her internship at LeDroit India.)

Scope of the Article

• Meaning of the Doctrine of Equivalents and why it exists alongside literal infringement
• Its origin and evolution under United States patent law
• The triple identity test and the all-elements rule
• Prosecution history estoppel as a limit on the doctrine
• Judicial treatment of the doctrine in India, from Raj Prakash to Sotefin SA
• Practical difficulties a patentee faces while proving infringement by equivalence
• The underlying tension between protecting patentees and giving the public fair notice of claim boundaries

Abstract

Patent claims are supposed to mark out the boundary of a patentee’s monopoly, but rather a boundary drawn in words rarely covers every way a competitor might try to work around it. The doctrine of equivalents allows a court to find infringement even when an accused product or process does not fall within the ambit of literal wording of a claim, provided the difference between the two is insubstantial.

This article entraces the doctrine from its American origins in Graver Tank via the tightening effect of Warner-Jenkinson and Festo, and there upon examines how Indian courts, working without any express statutory basis for the doctrine, have arrived at a similar result through the older idea of pith and marrow. It also looks at the practical alter of proving patent infringement by equivalence, and the also recurring difficulty of balancing a patentee’s genuine protection against the public’s right to rely on the words of a duly granted claim.

Keywords

Doctrine of Equivalents, Literal Infringement, Doctrine of Pith and Marrow, Prosecution History Estoppel, Patents Act 1970, Patent Infringement

Introduction

A patent is given for what is described and what is claimed in the patent specification, and infringement, in the ordinary sense, means someone else making, using or selling that exact thing without permission of the patentee. The issue is that patent claims are written in language, and language has confusions sometimes.

A competitor who understands partly what a patent covers can often change one part of it slightly, enough to fall outside the literal words of the claim, while still getting practically the same result the patent was meant to protect. If courts allowed this every time, the value of a patent would compromise very quickly, because anyone could copy the invention with minor cosmetic changes and call it a new product.

This is the gap that the doctrine of equivalents is to close. It lets a court hold a defendant liable for infringement even when the accused product or process does not match the claim word for word, so long as the differences between them are minor or ‘insubstantial’.That means,, if something does the same job, in  the same way, to get substantially the same result, courts have been willing to treat it as equivalent to the patented invention rather than as something genuinely different. This article looks at how that idea developed, where its limits lie, and how it has been received by courts in India, where the doctrine has no direct footing in the Patents Act, 1970.

Literal Infringement and Its Limits

Before getting into equivalence, it helps to be clear about what literal infringement means. A claim in a patent is broken down into its singular elements or limitations, and the accused product or process is compared against each of these elements one by one. If every element of the claim,word by word, is present in the accused product, infringement is literal and there is not much scope for argument. The problem arises when even one element is missing in its literal form, because under a strict reading, the claim would then not be infringed at all, no matter how close the accused product  comes to the patented invention.

This is where the all-or-nothing character of literal infringement starts to feel unfair to a patentee. A defendant who substitutes, even a slightly different chemical compound for the one named in the claim, without changing what the invention actually does, would escape liability purely on a technicality. Courts have recognised this problem early on, and the doctrine of equivalents grew out of the need to prevent what is  called a ‘fraud on the patent’  copying the substance of an invention while avoiding its exact words.

Origins of the Doctrine: The American Trajectory

The doctrine of equivalents is largely of origin of United States patent jurisprudence, and its start point was Winans v. Denmead (1854), where the Supreme Court held that a patentee should not lose protection simply because an infringer made an unimportant and insubstantial change to escape the literal claim language. That  idea looks to substance, not just form what has stayed at the centre of the doctrine ever since, even as the tests used to apply it have changed through the course  considerably.

Graver Tank and the ‘Triple Identity’ Test

The doctrine took its relevant modern shape in Graver Tank & Manufacturing Co. v. Linde Air Products Co., 339 U.S. 605 (1950). The patent that was in question covered a welding flux made up of using ‘alkaline earth metal silicates’. The defendant’s flux used magnesium silicate instead, which technically was not an alkaline earth metal. It was read literally, there was no infringement. The Supreme Court disagreed with a literal reading and therefore held that infringement can still be found if the substituted element performs substantially the same function, in substantially the same way, to achieve substantially the same result as the element it replaces. This has since been called the triple identity test. The Court also introduced the idea of ‘known interchangeability’  if it was already understood in the relevant field that the substitute could be swapped in for the claimed element, that supports a finding of equivalence.

What Graver Tank did, in effect, was shift the inquiry away from the dictionary meaning of the claim words and toward what the invention actually achieves. That is a sensible idea in principle, but it also opened the door to a fair amount of uncertainty, since ‘substantially the same’ is not a phrase with sharp edges.

Warner-Jenkinson and the All-Elements Rule

For nearly fifty years after Graver Tank, the Supreme Court did not revisit the doctrine directly, and lower courts applied it with varying degrees of generosity. This changed with Warner-Jenkinson Co. v. Hilton Davis Chemical Co., 520 U.S. 17 (1997), a case involving a patented ultrafiltration process defined partly by a pH range. The defendant’s process fell just outside the claimed range on one end, and the jury still found infringement under the doctrine of equivalents.

The Supreme Court used the case to reaffirm that the doctrine survives, but it tied the analysis down more firmly than Graver Tank had. The Court held that equivalence must be assessed element by element, not by treating the invention as one undivided whole. This is the all-elements rule: every single limitation of the claim, or its substantial equivalent, must be present in the accused product. A patentee cannot argue that the invention ‘as a whole’ is equivalent while ignoring the fact that one specific claim element is simply missing.

The practical effect of this is that the doctrine of equivalents is not at all a licence to expand a patent’s scope loosely. It is a narrow correction applied to individual claim elements where the literal wording falls just short, not a general permission to catch anything resembling the invention.

Festo and Prosecution History Estoppel

A major limit on the doctrine comes from the case of Festo Corp. V. Shoketsu Kinzoku Kogyo Kabushiki Co., 535 U.S. 722 (2002). During the process of getting a patent applicants often make their claims more specific when the patent office raises concerns. For example they do this to avoid inventions. The main question in the Festo case was what happens to the doctrine of equivalents after a claim has been made narrower in this way.

The Supreme Court decided that such a change usually leads to prosecution history estoppel. The patent owner is therefore assumed to have given up the area between the broader claim and the final narrower one that was approved. The patent owner cannot then use the doctrine of equivalents to take that area. The assumption can be challenged in few situations but the usual rule goes against the patent owner. From a fairness perspective if a patent owner made a claim narrower to get the patent the public should be able to rely on the scope without worrying that the patent owner will try to take back the larger area later.

These two are read together, Graver Tank, Warner-Jenkinson and Festo show a doctrine that started out fairly open-ended and was gradually reined in. Equivalence is still available, but only element by element, and only where the patentee has not already given up the relevant ground during prosecution.

The Position in India

The Patents Act, 1970 does not use the phrase ‘doctrine of equivalents’ anywhere, and Indian courts have generally worked around this using the older English concept of pith and marrow, which asks whether the essential substance of the invention has been taken, ignoring trivial or unessential variations. The end result is fairly close to the American doctrine, even though the reasoning path is different.

Raj Prakash v. Mangat Ram Chowdhry (1977)

Raj Prakash v. Mangat Ram Chowdhry is usually treated as the earliest Indian case dealing with non-literal infringement, even though the Delhi High Court did not use the words ‘doctrine of equivalents’. Relying on the Australian decision in Beecham Group Ltd. v. Bristol Laboratories Pvt. Ltd. (1968), the Court held that the pith and marrow of the invention has to be looked at, rather than getting caught up in the fine print of the specification. If the infringing product achieves the same object as the patented article through only trifling variations, that is still infringement.

Ravi Kamal Bali v. Kala Tech and Ors. (2008)

Ravi Kamal Bali v. Kala Tech and Ors., (2008) 38 PTC 435, is generally described as the first Indian judgment to engage with the doctrine of equivalents in more explicit terms. The dispute concerned tamper-proof locks and seals, and the plaintiff sought an interim injunction, arguing that the defendant’s product performed the same work, in substantially the same manner, to give the same output.

The Bombay High Court examined whether the two products shared the same purpose, the same nature of material, and the same underlying working principle. The interim injunction itself was not granted on the facts, but the case is significant because it was the first real judicial discussion of equivalence-based infringement in the Indian context, framed in language close to the American triple identity test.

TVS Motor Company Ltd. v. Bajaj Auto Ltd.

The Madras High Court in the case of TVS Motor Company Ltd. Versus Bajaj Auto Ltd. Used a method when interpreting the claims. The court decided that, in an infringement case the main thing is whether the person accused of infringement has taken the part of the invention. This is true even if there are things left out or added to the features that are claimed.This again therfore echoes the pith and marrow approach rather than a rigid and concrete word-for-word comparison.

Bishwanath Prasad Radhe Shyam v. Hindustan Metal Industries (1978)

The Supreme Court decision is widely known for its discussion of claim construction generally. The Supreme Court decision is relevant here because the Supreme Court decision held that claims must be read with the description of the invention and that a patentee cannot claim more than what was actually disclosed as the invention. This principle indirectly shapes how far the doctrine of equivalents can stretch in India. Equivalence can fill gaps between the claim and the accused product but equivalence cannot be used to claim something the specification never really supported.

Sotefin SA v. Indraprastha Cancer Society and Research Centre & Ors. (2022)

Sotefin SA v. Indraprastha Cancer Society and Research Centre & Ors. is one of the more detailed recent judgments on the subject. The dispute involved automated car-parking equipment, referred to as ‘Dollies’, and the Delhi High Court granted an interim injunction after finding that the defendants’ ‘Smart Dollies’ infringed the plaintiff’s patent. The Court went into the essential-elements analysis at some length, examining whether the allegedly infringing product incorporated each essential feature of the patented invention, and treated the doctrine of equivalents as very much a live and applicable principle under Indian law, even without express statutory backing.

A similar essential-elements approach appears in the string of cases between FMC Corporation v. Natco Pharma Ltd., decided by the Delhi High Court through 2022, concerning a patented insecticide process. Natco argued, among other things, that the doctrine of equivalents has only limited application to method or process patents. The Court did not accept this as a blanket rule and went on to examine whether the process actually used by Natco varied only insubstantially from the patented process.

These recent decisions suggest that Indian courts are becoming more comfortable applying a structured, element-by-element analysis rather than a loose ‘overall impression’ test, which brings Indian practice closer to the American position even though the Warner-Jenkinson case itself is never cited as binding authority.

Proving Infringement by Equivalence: Practical Difficulties

Winning a case on the doctrine of equivalents is harder than it might sound from the case law summaries above, for a few practical reasons.

• Burden of proof: The patentee has to lead specific, element-wise evidence, usually expert testimony on why a particular substitution is insubstantial. A general assertion that the products ‘look similar’ or ‘do the same thing’ is not enough after Warner-Jenkinson; the function-way-result comparison has to be made claim element by claim element.
• Claim drafting quality: Broadly and carefully drafted claims give more room for an equivalence argument later. Narrow, overly specific claims leave less to argue about, and any amendment made during prosecution can trigger estoppel and cut off equivalence entirely for that portion of the claim.
• Uncertainty for third parties: Competitors are meant to be able to read a patent and design around it with some confidence. A doctrine that extends protection beyond the literal words makes that exercise less predictable, which is exactly why courts, both in the US and in India, keep stressing that equivalence should be the exception and not the rule.
• Process versus product patents: As the Natco litigation shows, there is still some disagreement over how freely the doctrine should apply to method patents, where the steps themselves, and their sequence, may be treated as more central to the claim than they would be for a simple product patent.

One practical concern worth flagging is that none of this analysis is available in the abstract. Courts insist on comparing the actual accused product against the actual patent claims, element by element, supported by technical evidence. A patentee who goes to court hoping the doctrine of equivalents will rescue a badly drafted or overly narrow claim is usually disappointed.

Balancing Protection and Public Notice

The doctrine of equivalents sits at an uncomfortable but necessary point in patent law. Claims exist to give the public fair notice of what is off-limits, and if courts routinely expanded that boundary after the fact, nobody could rely on a patent’s wording with any confidence. At the same time, an infringer who makes a trivial, cosmetic change purely to dodge the literal claim language is doing exactly what patent law is meant to prevent.

Warner-Jenkinson’s all-elements rule and Festo’s prosecution history estoppel exist to keep this balance from tipping too far in the patentee’s favour, while Graver Tank and the Indian pith-and-marrow cases exist to stop it tipping too far the other way. Neither side of this balance is perfectly stable, and that is probably unavoidable given that the whole question turns on how ‘insubstantial’ a difference really is, which is ultimately a judgment call rather than something with a fixed formula.

Conclusion

The doctrine of equivalents fills a gap that a purely literal reading of patent claims would otherwise leave open. It began in the United States as a fairly broad, fact-driven enquiry in Graver Tank, was tightened by the all-elements rule in Warner-Jenkinson, and was tightened further by prosecution history estoppel in Festo.

India has reached a broadly similar place through its own route, starting from the pith-and-marrow approach in Raj Prakash and developing, through cases like Ravi Kamal Bali and more recently Sotefin SA and the FMC-Natco litigation, into a more structured, element-based analysis.

For a patentee, the real lesson from this line of cases is that the doctrine of equivalents is not a safety net for weak claim drafting it is a narrow tool for catching genuinely insubstantial variations, and proving that requires precise, element-wise evidence rather than a general sense that two products look alike.