Patent Infringement by Equivalents
The patent laws “promote the Progress of Science and useful Arts” by rewarding innovation with a temporary monopoly. The monopoly is a property right; and like any property right, its boundaries should be clear. A patent holder should know what he owns, and the public should know what he does not. For this reason, the patent laws require inventors to describe their work in “full, clear, concise, and exact terms,” as part of the delicate balance the law attempts to maintain between inventors, who rely on the promise of the law to bring the invention forth, and the public, which should be encouraged to pursue innovations, creations, and new ideas beyond the inventor’s exclusive rights.
Over 150 years ago, the Supreme Court expanded the potential scope of patents by adopting a doctrine to prevent “substantial copies” of an invention by providing coverage over inventions that are “equivalent” to that patented.
The doctrine of equivalents permits a finding of infringement even if the accused device or method does not literally fall within the scope of the construed patent claims. Instead, a device or method may infringe under the doctrine of equivalents if it performs “substantially the same function in substantially the same way to obtain the same result” as the patented invention. Thus, the doctrine of equivalents permits an expansion of patent rights beyond the literal scope of the patent claims. One purpose of the doctrine of equivalents is to protect patentees from those who seek “to evade liability for infringement by making only insubstantial changes to a patented invention.
POSITION IN THE UNITED STATES
The doctrine of equivalents is a legal rule that allows a court to hold a party liable for patent infringement even though the infringing device or process does not fall within the literal scope of a patent claim, but nevertheless is equivalent to the claimed invention.
Graver Tank & Mfg. Co., Inc. Et Al. V. Linde Air Products Co
Linde Air Products Co., owner of the Jones patent for an electric welding process and for fluxes to be used therewith, brought an action for infringement against Lincoln and the two Graver companies. The trial court held four flux claims valid and infringed and certain other flux claims and all process claims invalid. The Court of Appeals affirmed findings of validity and infringement as to the four flux claims but reversed the trial court and held valid the process claims and the remaining contested flux claims. Supreme Court granted certiorari, 335 U. S. 810, and reversed the judgment of the Court of Appeals insofar as it reversed that of the trial court, and reinstated the District Court decree . Rehearing was granted, limited to the question of infringement of the four valid flux claims and to the applicability of the doctrine of equivalents to findings of fact in this case. The single issue was whether the trial court’s holding that the four flux claims have been infringed will be sustained. Any issue as to the validity of these claims was unanimously determined by the previous decision in the Supreme Court and attack on their validity cannot be renewed then by reason of limitation on grant of rehearing. The disclosure, the claims, and the prior art have been adequately described in our former opinion and in the opinions of the courts below.
In determining whether an accused device or composition infringes a valid patent, resort must be had in the first instance to the words of the claim. If accused matter falls clearly within the claim, infringement is made out and that is the end of it. But 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.
The doctrine of equivalents evolved in response to this experience. The essence of the doctrine is that one may not practice a fraud on a patent. Originating almost a century ago in the case of Winans v. Denmead it has been consistently applied by the Courts. A patentee may invoke this doctrine to proceed against the producer of a device “if it performs substantially the same function in substantially the same way to obtain the same result.” Sanitary Refrigerator Co. v. Winter  . The theory on which it is founded is that “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 ,
In its early development, the doctrine was usually applied in cases involving devices where there was equivalence in mechanical components. Subsequently, however, the same principles were also applied to compositions, where there was equivalence between chemical ingredients. Today the doctrine is applied to mechanical or chemical equivalents in compositions or devices.
What constitutes equivalency must be determined against the context of the patent, the prior art, and the particular circumstances of the case. Equivalence, in the patent law, is not the prisoner of a formula and is not an absolute to be considered in a vacuum. It does not require complete identity for every purpose and in every respect. In determining equivalents, things equal to the same thing may not be equal to each other and, by the same token, things for most purposes different may sometimes be equivalents. Consideration must be given to the purpose for which an ingredient is used in a patent, the qualities it has when combined with the other ingredients, and the function which it is intended to perform.
The Court coming to the facts of the case that in the particular case there are two electric welding compositions or fluxes: the patented composition, Unionmelt Grade 20, and the accused composition, Lincoln weld 660. The patent under which Unionmelt is made claim essentially a combination of alkaline earth metal silicate and calcium fluoride; Unionmelt actually contain, however, silicates of calcium and magnesium, two alkaline earth metal silicates. Lincolnweld’s composition is similar to Unionmelt’s, except that it substitutes silicates of calcium and manganese—the latter not an alkaline earth metal—for silicates of calcium and magnesium. In all other respects, the two compositions are alike. The mechanical methods in which these compositions are employed are similar. They are identical in operation and produce the same kind and quality of weld.
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 the 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.
The Supreme Court observed that it was difficult to conceive of a case more appropriate for application of the doctrine of equivalents. The disclosures of the prior art made clear that manganese silicate was a useful ingredient in welding compositions. Specialists familiar with the problems of welding compositions understood that manganese was equivalent to and could be substituted for magnesium in the composition of the patented flux and their observations were confirmed by the literature of chemistry. Without some explanation or indication that Lincolnweld 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. Though infringement was not literal, the changes which avoid literal infringement are colorable only. The Supreme Court concluded that the trial court’s judgment of infringement respecting the four flux claims was proper.
Warner-Jenkinson Company v. Hilton Davis Chemical Co.
Petitioner Warner-Jenkinson Co. and respondent Hilton Davis Chemical Co. manufactured dyes. Impurities in those dyes must be removed. Hilton Davis held United States Patent No. 4,560,746 (`746 patent), which disclosed an improved purification process involving “ultra filtration.” The `746 process filter impure dye through a porous membrane at certain pressures and pH levels, resulting in a high purity dye product. The `746 patent issued in 1985.
As relevant to this case, the patent claims as its invention an improvement in the ultra filtration process as follows:
“In a process for the purification of a dye . . . the improvement which comprises: subjecting an aqueous solution . . . to ultra filtration through a membrane having a nominal pore diameter of 5-15 Angstroms under a hydrostatic pressure of approximately 200 to 400 p.s.i.g., at a pH from approximately 6.0 to 9.0, to thereby cause separation of said impurities from said dye . . . .”
The inventors added the phrase “at a pH from approximately 6.0 to 9.0” during patent prosecution. At a minimum, this phrase was added to distinguish a previous patent (the “Booth” patent) that disclosed an ultrafiltration process operating at a pH above 9.0. The parties disagree as to why the low-end pH limit of 6.0 was included as part of the claim. In 1986, Warner-Jenkinson developed an ultrafiltration process that operated with membrane pore diameters assumed to be 5-15 Angstroms, at pressures of 200 to nearly 500 p. s. i. g., and at a pH of 5.0. Warner-Jenkinson did not learn of the `746 patent until after it had begun commercial use of its ultrafiltration process. Hilton Davis eventually learned of Warner-Jenkinson’s use of ultrafiltration and, in 1991, sued Warner-Jenkinson for patent infringement.
Hilton Davis conceded that there was no literal infringement, and relied solely on the doctrine of equivalents. Over Warner-Jenkinson’s objection that the doctrine of equivalents was an equitable doctrine to be applied by the court, the issue of equivalence was included among those sent to the jury. The jury found that the `746 patent was not invalid and that Warner-Jenkinson infringed upon the patent under the doctrine of equivalents. The District Court entered a permanent injunction prohibiting Warner-Jenkinson from practicing ultrafiltration below 500 p. s. i. g. and below 9.01 pH. A fractured en banc Court of Appeals for the Federal Circuit affirmed .
Supreme Court granted certiorari, 516 U. S. 1145 (1996). 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.
In this case, the patent examiner objected to the patent claim due to a perceived overlap with the Booth patent, which revealed an ultrafiltration process operating at a pH above 9.0. In response to this objection, the phrase “at a pH from approximately 6.0 to 9.0” was added to the claim. While it is undisputed that the upper limit of 9.0 was added in order to distinguish the Booth patent, the reason for adding the lower limit of 6.0 is unclear. The lower limit certainly did not serve to distinguish the Booth patent, which said nothing about pH levels below 6.0. Thus, while a lower limit of 6.0, by its mere inclusion, became a material element of the claim, that did not necessarily preclude the application of the doctrine of equivalents as to that element. Where the reason for the change was not related to avoiding the prior art, the change may introduce a new element, but it does not necessarily preclude infringement by equivalents of that element
Supreme Court observed that a holding that certain reasons for a claim amendment may avoid the application of prosecution history estoppel was not tantamount to holding that the absence of a reason for an amendment may similarly avoid such an estoppel. It suggested to place the burden on the patent holder to establish the reason for an amendment required during patent prosecution. The court then would decide whether that reason is sufficient to overcome prosecution history estoppel as a bar to application of the doctrine of equivalents to the element added by that amendment. Where no explanation is established, however, the court should presume that the patent applicant had a substantial reason related to patentability for including the limiting element added by amendment. In those circumstances, prosecution history estoppel would bar the application of the doctrine of equivalents as to that element. Prosecution history estoppel place reasonable limits on the doctrine of equivalents, and further insulate the doctrine from any feared conflict with the Patent Act. The Supreme Court reasoned that as the respondent had not proffered a reason for the addition of a lower pH limit, it was impossible to tell whether the reason for that addition could properly avoid an estoppel. It remanded back to the Federal Circuit to consider whether reasons for that portion of the amendment were offered or not and whether further opportunity to establish such reasons would be proper.
The Supreme Court concluded that the determination of equivalence should be applied as an objective inquiry on an element-by-element basis. Prosecution history estoppel continue to be available as a defense to infringement, but if the patent holder demonstrated that an amendment required during prosecution had a purpose unrelated to patentability, a court must consider that purpose in order to decide whether an estoppel was precluded.
Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., Ltd. 
Petitioner Festo Corporation owned two patents for an improved magnetic rodless cylinder, a piston-driven device that relies on magnets to move objects in a conveying system. The device had many industrial uses and has been employed in machinery as diverse as sewing equipment and the Thunder Mountain ride at Disney World. Although the precise details of the cylinder’s operation are not essential here, the prosecution history must be considered.
Petitioner’s patent applications, as often occurs, were amended during the prosecution proceedings. The application for the first patent, the Stoll Patent (U. S. Patent No. 4,354,125), was amended after the patent examiner rejected the initial application because the exact method of operation was unclear and some claims were made in an impermissible way. (They were multiply dependent.) 35 U. S. C. § 112 (1994 ed.). The inventor, Dr. Stoll, submitted a new application designed to meet the examiner’s objections and also added certain references to prior art. 37 CFR § 1.56 (2000). The second patent, the Carroll Patent (U. S. Patent No. 3,779,401), was also amended during a reexamination proceeding. The prior art references were added to this amended application as well. Both amended patents added a new limitation—that the inventions contain a pair of sealing rings, each having a lip on one side, which would prevent impurities from getting on the piston assembly. The amended Stoll Patent added the further limitation that the outer shell of the device, the sleeve, be made of a magnetizable material.
After Festo began selling its rodless cylinder, respondents (whom we refer to as SMC) entered the market with a device similar, but not identical, to the ones disclosed by Festo’s patents. SMC’s cylinder, rather than using two one-way sealing rings, employs a single sealing ring with a two-way lip. Furthermore, SMC’s sleeve is made of a non magnetizable alloy. SMC’s device does not fall within the literal claims of either patent, but petitioner contends that it is so similar that it infringes under the doctrine of equivalents.
SMC contended that Festo was estopped from making this argument because of the prosecution history of its patents. The sealing rings and the magnetized alloy in the Festo product were both disclosed for the first time in the amended applications. In SMC’s view, these amendments narrowed the earlier applications, surrendering alternatives that are the very points of difference in the competing devices—the sealing rings and the type of alloy used to make the sleeve. As Festo narrowed its claims in these ways in order to obtain the patents, says SMC, Festo is now estopped from saying that these features are immaterial and that SMC’s device is an equivalent of its own.
The United States District Court for the District of Massachusetts disagreed. It held that Festo amendments were not made to avoid prior art, and therefore the amendments were not the kind that gave rise to estoppel. A panel of the Court of Appeals for the Federal Circuit affirmed . Supreme Court granted certiorari, vacated, and remanded in light of our intervening decision in Warner-Jenkinson. v. Hilton Davis Chemica  . After a decision by the original panel on remand, 172 F. 3d 1361 (1999), the Court of Appeals ordered rehearing en banc.
The en banc court reversed, holding that prosecution history estoppel barred Festofrom asserting that the accused device infringed its patents under the doctrine of equivalents . The court held, with only one judge dissenting, that estoppel arises from any amendment that narrows a claim to comply with the Patent Act, not only from amendments made to avoid prior art. More controversial in the Court of Appeals was its further holding: When estoppel applies, it stands as a complete bar against any claim of equivalence for the element that was amended. Id., at 574-575. The court acknowledged that its own prior case law did not go so far. Previous decisions had held that prosecution history estoppel constituted a flexible bar, foreclosing some, but not all, claims of equivalence, depending on the purpose of the amendment and the alterations in the text. The court concluded, however, that its precedents applying the flexible-bar rule should be overruled because this case-by-case approach has proved unworkable. In the court’s view a complete-bar rule, under which estoppel bars all claims of equivalence to the narrowed element, would promote certainty in the determination of infringement cases.
The Supreme Court held that It is true that the doctrine of equivalents renders the scope of patents less certain. It may be difficult to determine what is, or is not, an equivalent to a particular element of an invention. If competitors cannot be certain about a patent’s extent, they may be deterred from engaging in legitimate manufactures outside its limits, or they may invest by mistake in competing products that the patent secures. In addition the uncertainty may lead to wasteful litigation between competitors, suits that a rule of literalism might avoid. These concerns with the doctrine of equivalents, however, are not new. Each time the Court has considered the doctrine, it has acknowledged this uncertainty as the price of ensuring the appropriate incentives for innovation, and it has affirmed the doctrine over dissents that urged a more certain rule.
Commenting upon Prosecution history estoppel, the Supreme Court observed that it requires that the claims of a patent be interpreted in light of the proceedings in the PTO during the application process. Estoppel is a “rule of patent construction” that ensures that claims are interpreted by reference to those “that have been cancelled or rejected.” Schriber-Schroth Co. v. Cleveland TrustCo . The doctrine of equivalents allows the patentee to claim those insubstantial alterations that were not captured in drafting the original patent claim but which could be created through trivial changes. When, however, the patentee originally claimed the subject matter alleged to infringe but then narrowed the claim in response to a rejection, he may not argue that the surrendered territory comprised unforeseen subject matter that should be deemed equivalent to the literal claims of the issued patent.
Prosecution history estoppel ensures that the doctrine of equivalents remains tied to its underlying purpose. Where the original application once embraced the purported equivalent but the patentee narrowed his claims to obtain the patent or to protect its validity, the patentee cannot assert that he lacked the words to describe the subject matter in question. The doctrine of equivalents is premised on language’s inability to capture the essence of innovation, but a prior application describing the precise element at issue undercuts that premise. In that instance the prosecution history has established that the inventor turned his attention to the subject matter in question, knew the words for both the broader and narrower claim, and affirmatively chose the latter.
Petitioner concede that the limitations at issue—the sealing rings and the composition of the sleeve—were made for reasons related to § 112, if not also to avoid the prior art. Prosecution history estoppel arise when a claim is narrowed to comply with § 112, but it give rise to the second question presented: Does the estoppel bar the inventor from asserting infringement against any equivalent to the narrowed element or might some equivalents still infringe? The Court of Appeals held that prosecution history estoppel is a complete bar, and so the narrowed element must be limited to its strict literal terms. Based upon its experience the Court of Appeals decided that the flexible-bar rule is unworkable because it leads to excessive uncertainty and burdens legitimate innovation. The Supreme Court observed that though prosecution history estoppel can bar a patentee from challenging a wide range of alleged equivalents made or distributed by competitors, its reach require an examination of the subject matter surrendered by the narrowing amendment. The complete bar avoid this inquiry by establishing a per se rule; but that approach is inconsistent with the purpose of applying the estoppel in the first place—to hold the inventor to the representations made during the application process and to the inferences that may reasonably be drawn from the amendment. By amending the application, the inventor is deemed to concede that the patent does not extend as far as the original claim. It does not follow, however, that the amended claim becomes so perfect in its description that no one could devise an equivalent. After amendment, as before, language remains an imperfect fit for invention. The narrowing amendment may demonstrate what the claim is not; but it may still fail to capture precisely what the claim is. There is no reason why a narrowing amendment should be deemed to relinquish equivalents unforeseeable at the time of the amendment and beyond a fair interpretation of what was surrendered. Nor is there any call to foreclose claims of equivalence for aspects of the invention that have only a peripheral relation to the reason the amendment was submitted. The amendment does not show that the inventor suddenly had more foresight in the drafting of claims than an inventor whose application was granted without amendments having been submitted. It shows only that he was familiar with the broader text and with the difference between the two. As a result, there is no more reason for holding the patentee to the literal terms of an amended claim than there is for abolishing the doctrine of equivalents altogether and holding every patentee to the literal terms of the patent.
Just as Warner-Jenkinson held that the patentee bears the burden of proving that an amendment was not made for a reason that would give rise to estoppel, the Supreme Court held here that the patentee should bear the burden of showing that the amendment does not surrender the particular equivalent in question. The patentee, as the author of the claim language, may be expected to draft claims encompassing readily known equivalents. A patentee’s decision to narrow his claims through amendment may be presumed to be a general disclaimer of the territory between the original claim and the amended claim. Exhibit Supply, 315 U. S., at 136-137 (“By the amendment [the patentee] recognized and emphasized the difference between the two phrases and proclaimed his abandonment of all that is embraced in that difference”). There are some cases, however, where the amendment cannot reasonably be viewed as surrendering a particular equivalent. The equivalent may have been unforeseeable at the time of the application; the rationale underlying the amendment may bear no more than a tangential relation to the equivalent in question; or there may be some other reason suggesting that the patentee could not reasonably be expected to have described the insubstantial substitute in question. In those cases the patentee can overcome the presumption that prosecution history estoppel bars a finding of equivalence.
POSITION IN THE UNITED KINGDOM
Catnic Components Ltd. v. Hill & Smith Ltd. 
House of Lords
Catnic Components had a patent for a lintel, used to provide structural support over a door or window opening in a brick wall. Part of the specification required a bar to “extend vertically”. Hill created a virtually identical invention that had a bar that extended at an upwards slant, only 6 degrees from being completely vertical. Despite the difference the device worked entirely in the same way as Catnic’s invention.
Catnic sued for patent infringement. At trial, the judge held there was an infringement under the “pith and marrow” doctrine. The Court of Appeal overturned the ruling as although it held that the “vertical” requirement was an exact and essential element of the patent, the effect did not change. The court affirmed the use of purposive construction to patent interpretation and found an infringement. The House of Lords held that a patent specification is a unilateral statement by the patentee, in words of his own choosing, addressed to those likely to have a practical interest in the subject matter of his invention (i.e. “skilled in the art”), by which he informs them what he claims to be the essential features of the new product or process for which the letters patent grant him a monopoly. It is called “pith and marrow” of the claim. A patent specification should be given a purposive construction rather than a purely literal one derived from applying to it the kind of meticulous verbal analysis in which lawyers are too often tempted by their training to indulge. The question in each case is: whether persons with practical knowledge and experience of the kind of work in which the invention was intended to be used, would understand that strict compliance with a particular descriptive word or phrase appearing in a claim was intended by the patentee to be an essential requirement of the invention so that any variant would fall outside the monopoly claimed, even though it could have no material effect upon the way the invention worked.
The question, of course, does not arise where the variant would in fact have a material effect upon the way the invention worked. Nor does it arise unless at the date of publication of the specification it would be obvious to the informed reader that this was so. Where it is not obvious, in the light of then-existing knowledge, the reader is entitled to assume that the patentee thought at the time of the specification that he had good reason for limiting his monopoly so strictly and had intended to do so, even though subsequent work by him or others in the field of the invention might show the limitation to have been unnecessary. It is to be answered in the negative only when it would be apparent to any reader skilled in the art that a particular descriptive word or phrase used in a claim cannot have been intended by the patentee, who was also skilled in the art, to exclude minor variants which, to the knowledge of both him and the readers to whom the patent was addressed, could have no material effect upon the way in which the invention worked
House of Lords further observed that :
“… both parties to this appeal have tended to treat `textual infringement’ and infringement of the ‘pith and marrow’ of an invention as if they were separate causes of action, the existence of the former to be determined as a matter of construction only and of the latter upon some broader principle of colourable invasion. There is, in my view, no such dichotomy; there is but a single cause of action and to treat it otherwise, particularly in cases like that which is the subject of the instant appeal, is liable to lead to confusion.”
Article 69 of the European Patent Convention
Extent of protection
(1) The extent of the protection conferred by a European patent or a European patent application shall be determined by the claims. Nevertheless, the description and drawings shall be used to interpret the claims.
(2) For the period up to grant of the European patent, the extent of the protection conferred by the European patent application shall be determined by the claims contained in the application as published. However, the European patent as granted or as amended in opposition, limitation or revocation proceedings shall determine retroactively the protection conferred by the application, in so far as such protection is not thereby extended.
A protocol to the Convention attempts to define what this means. The protocol reads as follows:
“Article 69 shall not be interpreted in the sense that the extent of the protection conferred by a European patent is to be understood as that defined by the strict literal meaning of the wording used in the claims, the description and drawings being employed only for the purpose of resolving an ambiguity in the claims. Neither should it be interpreted in the sense that the claims serve only as a guideline and that the actual protection conferred may extend to what, from a consideration of the description and drawings by a person skilled in the art, the patentee contemplated. On the contrary it is to be interpreted as defining a position between these extremes which combines a fair protection for the patentee with a reasonable degree of certainty for third parties”.
There was a debate whether the purposive construction test in the Catnic case in fact complied with the protocol to Article 69 of the European Patent Convention. The doubt was finally cleared by the House of Lords in Kirin-Amgen v Hoechst Marion Roussel  . It held that the Protocol is a Protocol for the construction of article 69 and does not expressly lay down any principle for the construction of claims. It does say what principle should not be followed, namely the old English literalism, but otherwise it says only that one should not go outside the claims. It does however say that the object is to combine a fair protection for the patentee with a reasonable degree of certainty for third parties. It held that the claims must be construed in a way which attempts, so far as is possible in an imperfect world, not to disappoint the reasonable expectations of either side. What principle of interpretation would give fair protection to the patentee? Surely, a principle which would give him the full extent of the monopoly which the person skilled in the art would think he was intending to claim. And what principle would provide a reasonable degree of protection for third parties? Surely again, a principle which would not give the patentee more than the full extent of the monopoly which the person skilled in the art would think that he was intending to claim. Indeed, any other principle would also be unfair to the patentee, because it would unreasonably expose the patent to claims of invalidity on grounds of anticipation or insufficiency.
The House of Lords thus declared that the Catnic principle of construction was therefore precisely in accordance with the Protocol. It is intended to give the patentee the full extent, but not more than the full extent, of the monopoly which a reasonable person skilled in the art, reading the claims in context, would think he was intending to claim.
POSITION IN INDIA
In India, the doctrine of equivalents was discussed in Ravi Kamal Bali vs Kala Tech And Ors Though the matter was regarding an interim application for request of submission of additional documents by the defendant, yet in the order the Court observed that the counsel of the plaintiff submitted that while considering the question of infringement of patents, the Court ought to apply the doctrine of equivalence by which a device is set to infringe a claim if it “performs substantially the same function in substantially the same way to obtain the same result”. The test is whether the Defendants product appears to have taken the essence or what is sometimes called the pith and marrow of the invention. Counsel for the plaintiff further submitted that even under the Indian Patent Act, 1970 while deciding the question of infringement of patents, the Court ought to apply the doctrine of equivalence under which the Court must determine and distinguish the essential and nonessential elements of the product. He submitted that it is not necessary that the infringing goods must be identical in every respect to the patented goods and it is sufficient if it is found that what has been taken is the essence of the invention.
There seems to be many approaches towards dealing with non – literal infringement across different jurisdictions. Doctrine of equivalent and doctrine of Pith and Marrow are two such. In India cases have been very few where Doctrine of equivalents have been discussed. The debate between the doctrine of equivalents and promissory history estoppels will continue to come before the courts. It is advisable that there should be a single approach to tackle non literal infringement among all jurisdictions. But in recent years, distinguished academics have predicted the so-called “demise” of the doctrine of equivalents.
 Winans v. Denmead, 56 U.S. 330 (1853).
 339 U.S. 605 (1950)
 167 F. 2d 531.
 336 U. S. 271.
 280 U. S. 30, 42
 97 U. S. 120, 125
 520 U.S. 17 (1997)
 62 F. 3d 1512 (1995)
 535 U.S. 722 (2002)
 72 F. 3d 857 (1995).
 520 U. S. 17 (1997)
 234 F. 3d 558 (2000).
 311 U. S. 211, 220-221 (1940)
 (1982) R.P.C. 183
 (2004)UKHL 46
 2008(110)Bom LR 2167