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. 
A patent contains several parts—a specification, usually one or more drawings, and always one or more claims. No matter how much a questioned machine, manufacture, composition of matter or process may look like the specification and drawings of a patent, it is only the claims of the patent which can be infringed. For that reason, if an issue of infringement arises, it becomes necessary to examine the claims of the patent in question.
The first step is to "read" each claim of the patent upon the accused structure or process. Every requirement of each claim must be considered to see if each thing set out in the claim also appears in the accused practice. If one or more things set forth in a claim is not present in the practice being reviewed, there is no infringement of that claim. On the other hand, if each thing which is set out in even one claim of the patent is present in the accused structure or process, then there is direct and literal infringement. When literal infringement is found, that is normally the end of the inquiry.

An infringement analysis determines whether a claim in a patent literally "reads on" an accused infringer's device or process, or covers the allegedly infringing device under the doctrine of equivalents. The steps in the analysis are:

1. Construe the scope of the "literal" language of the claims.

2. Compare the claims, as properly construed, with the accused device or process, to determine whether there is literal infringement.

3. If there is no literal infringement, construe the scope of the claims under the doctrine of equivalents.

The doctrine of equivalents is an equitable doctrine which effectively expands the scope of the claims beyond their literal language to the true scope of the inventor's contribution to the art. However, there are limits on the scope of equivalents to which the patent owner is entitled.

Over 150 years ago, the  US Supreme Court in case of Winans v. Denmead , 56 U.S. 330 (1853), 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.


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 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.

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 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.


Catnic Components Ltd. v. Hill & Smith Ltd (1982) R.P.C. 183 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.
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."
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.
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. 

In India, the doctrine of equivalents was discussed in Ravi Kamal Bali vs Kala Tech And Ors 2008(110)Bom LR 2167, 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.
Ravi Kamal Bali, the plaintiff, instituted an infringement suit seeking an interim injunction inter alia restraining Kala Tech, the defendant, from making, using, selling or distributing tamper proof locks/seals falling within the scope of the claims of the his patent bearing No. 162675 and patent of addition No. 178879. Ravi Kamal alleged that the product of Kala Tech has similar constructional and functional features to his patented invention. Arguing further, Ravi Kamal contended that Kala Tech’s product do the same work, in substantially the same way and accomplishes substantially the same result thereby contributing to the infringement. In response, Kala Tech’s counsel while asserting on the differences in the constructional and functional structure between the two products, made a counter argument comparing the inventive step between the plaintiff’s patent and the patent of addition and the patents of plaintiff and defendant, and submitted that when the only difference between the plaintiff’s patent and the patent of addition is the addition of ‘vanes’ in the patent of addition then the defendant’s invention would also constitute a new invention qua the invention that is patented as the difference between the defendants invention and the plaintiff’s patented invention is greater than the difference between the patent of addition and the patent itself.
The submission above equated a patent with a patent of addition which was found erroneous by the Court on the ground that this would obliterate the rights of a patentee. The patent holders of the main invention and the patent of addition being different in the instance of the patent over the main invention being revoked may see the patent of addition continuing for the remainder of the term for the patent of the main invention, unless also applied for and revoked simultaneously. However, the Court denied the relief of an interim injunction justifying it on the plaintiff’s act of seeking an ad interim injunction on a representation which was incorrect in material aspect.
This case is important as it discusses Doctrine of equivalents, an important legal principle under the patent law regime covering indirect infringement. In addition to literal infringement which is direct and unambiguous, an indirect infringement takes place when insubstantial or minor changes made to the patented product or process causes it to fall outside the declared scope of the patent i.e. patent claims, but, which in practical terms, remains a duplicate of the patented product/process.

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.

Abhinav kr. Mishra