In Canadian patent infringement or validity litigation, the patent claims in issue must first be construed in order to ascertain the meaning of words in the claims as well as the scope of the claims. It is only once this step has been completed that the court can then go on to determine whether the claims are valid and/or have been infringed.
Over the years, Canadian courts have developed a number of principles that govern the construction of patents. One of these principles, namely claim differentiation, is discussed in this article.
At its simplest, the principle of claim differentiation provides that there is a rebuttable presumption that no two claims in a patent are redundant. In other words, different claims have different meanings. Depending on the circumstances, the application of this principle may favour the patentee or it may favour the alleged infringer.
In Halford v. Seed Hawk Inc, 2004 FC 88 rev’d in part on other grounds 2006 FCA 275 (“Halford”), Justice Pelletier described the principle of claim differentiation as follows:
- Interpretation of a disputed claim term requires reference not only to the specification but also to other claims;
- an independent claim cannot be given a construction which is inconsistent with the claims which are dependent upon it;
- where some claims are broad and others narrow, the narrow claim limitations cannot be read into the broad whether to avoid invalidity or to escape infringement;
- claim differentiation is a rebuttable presumption; and
- the presumption is especially strong when the limitation in dispute is the only meaningful difference between an independent and a dependent claim, and one party is arguing that the limitation in the dependent claim should be read into the independent claim.
In ABB v. Hyundai Heavy Industries 2013 FC 947 aff’d 2015 FCA 181, Justice Barnes discussed the application of the principle in circumstances where an essential feature is described in different claims using different terms:
 Forming a “reasonable view” of patent language does not, to my thinking, prefer any arguable interpretation that would uphold the patent. In most cases the language of the patent, when viewed contextually and objectively, will be sufficient to establish what was intended thereby ensuring the attainment of its purpose. I would add that the purposive approach is not an invitation to the Court to ignore the ordinary rules of grammar and syntax. If an essential feature of a patent is defined in a specific way and a different more expansive term is also introduced that can include the specific term, one would not generally interpret the two terms as denoting the same thing. The usual purpose of using different words is to distinguish one feature from another and not to express synonymy.
Reasons for Applying the Principle
The principle of claim differentiation is typically applied in patent litigation in order to determine the meaning of certain claim terms and to determine the scope of a particular claim or claims.
For example, a patentee who is seeking a broad construction of an independent claim in order to capture the defendant’s accused device may argue that a particular term in the claim must be construed more broadly than a similar term in a dependent claim so as to avoid a redundancy.
Similarly, a defendant who is seeking invalidate a patent claim may argue that the principle should be applied so as to give a broad construction to the claim that will then, in turn, render it invalid to the extent that its scope includes known, obvious or inoperable embodiments.
What Does a Term Mean?
In practice, close analysis of claims is usually required in order to determine whether a difference in language between two or more claims is meaningful and, if so, how that difference should be used to construe the claims. This analysis is, in most cases, the subject of competing expert testimony at trial and it is left to the trial judge to resolve the conflict.
For example, in Human Care v. Evolution Technologies 2018 FC 1302, Justice Elliott commented as follows in preferring the evidence of one expert over another:
 Keeping these principles in mind and on considering the evidence, I prefer the interpretation of Dr. Brienza. It not only preserves the language of the claims but also respects the principle of claim differentiation. In Claims 16 and 18, the location of the “tension rod means” is not specified but in Claims 1-9 it extends “between a bottom of the first support and a bottom of the second support.” Dr. Brienza put forward an interpretation that respects and gives meaning to that different claim language.
Similarly, in MIPS AB v. Bauer Hockey 2018 FC 485, the trial judge used claim differentiation to construe two claims. Justice Gagné commented as follows:
 The principle of claim differentiation dictates that “the starting assumption must be that claims are not redundant” (Ratiopharm Inc v Canada (Health), 2007 FCA 83 at para 33). If a limitation cannot be read into a claim to avoid infringement (Halford, above at paras 91-96), it can no more be read in to inflate the inventive concept disclosed in a claim in order to avoid invalidity for obviousness. Claim differentiation is a rebuttable presumption, but a strong one when the limitation found in the dependent claim is the only meaningful difference between an independent and a dependent claim…
The principle was also applied by Justice St-Louis in Eli Lilly v. Apotex 2020 FC 814 (“Eli Lilly”). Justice St-Louis commented as follows when applying the principle of claim differentiation to independent claim 1 and dependent claim 4:
 Apotex argues that the use of the indefinite article “a”, rather than the definite article “the”, indicates the patentee referred, in Claim 1, to a method of preparing both cis-diastereomers, ie the R,R and the S,S configurations, and did not limit Claim 1 to the R,R cis-diastereomer configuration. Apotex relies on (1) the usual meaning of an indefinite article whereby it refers to the non-specific noun that follows; (2) the fact that Claim 1b does not specify the type of tryptophan that must be used, which serves to confirm that the cis-diastereomer could be either the R,R or the S,S variant since different types of tryptophan would result in different configurations, showing no precise stereochemistry; and (3) the fact that Claim 4, which depends on Claim 1, narrows the tryptophan in step b to the D-tryptophan, which would lead to the R,R configuration, which, applying the principle of claim differentiation, directs us to conclude that Claim 1 is not limited to the R,R configuration.
 The Court must strive for a construction that is not redundant (Camso Inc v Soucy International Inc, 2019 FC 255 at paras 103, 186–190).
 Even if I adopt Lilly’s position in regards to the proper use of an indefinite article, as directed by the Patent Office, thus accepting that the desired cis-diastereomer was not properly introduced in the disclosure, I cannot rally to their overall position that Claim 1 only refers to the R,R cis configuration.
 My decision does not rely on grammar, but on the fact that (1) Claim 1b does not specify the type of tryptophan that must be used, which serves to confirm that the cis-diastereomer could be either the R,R or the S,S variant since different types of tryptophan would result in different configurations, showing no precise stereochemistry; and (2) Claim 4 , which is dependant on Claim 1, narrows the tryptophan in step b to the D-tryptophan, which leads to the R,R configuration, which, applying the principle of claim differentiation, directs the Court to strive for a construction of Claim 1 that is not redundant.
 This leads me to conclude that Claim 1 is directed to a desired cis-diastereomer, without specifically referring to the R,R configuration. This precise R,R configuration is achieved by the use of the D-tryptophan at Claim 4.
The Federal Court of Appeal also applied the principle of claim differentiation in Tetra Tech EBA Inc. v. Georgetown Rail Equipment Company 2019 FCA 203 where it held as follows:
 Georgetown argues that claim 8 aids in construing claim 7 of the Patent.
 Claim 8 is as follows:
The method of claim 7 further comprising (e) determining a tilt correction factor.
 Georgetown argues that the principle of claim differentiation presumes that patent claims are drafted so as not to be redundant. Thus, different claims have different scopes. Independent claims are understood to include more than dependent claims; a limitation in the dependent claim should not be read into the independent claim.
 Dependent claim 8 adds a limitation to independent claim 7: a tilt correction factor must be determined. It follows that determining a tilt correction factor is not a necessary element of claim 7.
 Therefore, I accept Georgetown’s submission that the method taught and claimed in claim 7 does not require the determination of a tilt correction factor. The “actual delta” referred to in claim 7 is simply the distance or difference between two points.
Of course, it is not every case where a trial judge concludes that claim differentiation has the effect sought by the party seeking to invoke the doctrine. For example, in Packers Plus v. Essential Energy 2017 FC 1111, the defendants sought to apply the principle in construing an independent claim directed to a method of fracturing a wellbore involving the use of sliding sleeves. Whereas the independent claim did not specify that the sliding sleeves must be closed, one of its dependent claims introduced such a requirement. The defendants therefore argued that claim differentiation mandated that the independent claim be construed so as to include both open and closed sleeves. Since the method only worked if the tubing string was run into the wellbore with the sleeves closed, the independent claim would necessarily be invalid for including within its scope methods that would not work.
Justice O’Reilly, however, disagreed with the defendants’ argument on the basis that a skilled reader of the patent would understand that the claimed method would only work if the sleeves were closed. As such, even though the reader would construe the independent claim as including methods where the tubing string was run into the wellbore with the sleeves open, they would understand that a means of closing the sleeves before fracturing the wellbore would be required.
What is an Essential Element?
Another manner in which the principle of claim differentiation can be used is in the determination of what is an essential element of a claim.
Canadian patent law requires that a claim be given a “purposive construction”. During purposive construction, the elements of a claim are identified as either essential elements (where substitution of another element or omission of the element takes the accused device outside of the monopoly) or non-essential elements (where substitution of omission of the element is not necessarily fatal to an allegation of infringement).
In Camso Inc. v. Soucy International Inc. 2019 FC 255 aff’d 2020 FCA 183, Justice Locke applied the principle of claim differentiation to the consideration of what constituted an essential element:
It is well understood that where one claim differs from another in only a single feature it is difficult to argue that the different feature has not been made essential to the claim…
Exceptions to the Principle
Although the presumption of claim differentiation has been broadly applied, given that it is a rebuttable presumption Canadian courts have on occasion declined to give different meanings to substantially identical claims.
For example, in Western Oilfield Rentals v. M-I LLC 2021 FCA 24, the appellant argued that the trial judge erred in construing the term “degassing chamber” in three claims as having the same scope as the term “chamber” in two other claims. In dismissing this argument, the Federal Court of Appeal held as follows:
The appellants argue that this conclusion failed to recognize the principle of claim differentiation and claim 17, which differs from claim 12 only in defining the chamber to be a degassing chamber. This argument cannot succeed. Firstly, the principle of claim differentiation is not absolute. Rather, it contemplates a rebuttable presumption: where one claim differs from another in only a single feature it is difficult to argue that the different feature has not been made essential to the claim: Whirlpool at para. 79; Camso at para. 103. Moreover, the appellants’ argument conflicts with the opinion of their own expert Mr. Murphy who construed the two terms as consistent with one another…
In Bauer Hockey v. Sport Maska 2020 FC 624, Justice Grammond discussed two scenarios in which the presumption of claim differentiation might be rebutted. The first scenario involved a situation where the wording of two claims might be attributable to sloppiness on the part of the patent agent rather than being evidence of the inventor’s intention to claim his or her invention in a particular manner. The other scenario involved what Justice Grammond described as “unscrupulous patentees” deliberately crafting two claims containing synonyms for a single feature:
 Of course, the application of this principle assumes a high degree of consistency in the drafting of the legal document under consideration. Even with respect to legislation, which is usually drafted with great care, the presumption against redundancy may be rebutted if other clues point in the opposite direction: Schreiber v Canada (Attorney General), 2002 SCC 62 at paragraphs 74-75,  3 SCR 269. In contractual matters, the presumption has little strength where the contract was drafted in haste, without the assistance of legal professionals: Bélisle v Marcotte,  Que QB 46 at 49. In his closing submissions, counsel for Bauer suggested that patent agents typically have a quite limited budget for preparing a patent application. Thus, they would not have the degree of understanding of prior art that one gains through litigation. This would explain why patents often include a string of dependent claim in a strategy to obtain the widest protection possible while avoiding invalidity. Thus, one should not be surprised that the claims differentiation principle has, too, been characterized as a “rebuttable presumption:” Halford v Seed Hawk Inc, 2004 FC 88 at paragraph 94. It “cannot overcome a purposive interpretation” Bridgeview Manufacturing Inc v 931409 Alberta Ltd (Central Alberta Hay Centre), 2010 FCA 188 at paragraph 33.
 In reality, if claims differentiation were a hard-and-fast rule instead of a rebuttable presumption, unscrupulous patentees could be tempted to include in their applications a dependent claim that would cover, as its additional feature, something that is merely a synonym of a feature of the independent claim. If the stratagem were to go undetected and the patent issues, the patentee could then rely on the claims differentiation principle to argue that the independent claim must cover more than what was contemplated when it applied for the patent, typically embracing its competitors’ products ex post facto. Courts do not allow themselves to be so cornered. This is one reason why interpretive principles remain flexible.
It will be apparent from the above discussion of the case law that the principle of claim differentiation should be front and centre during the drafting of any patent application.
For example, all claims should be reviewed not only on their own but also relative to one another to ensure that a limitation introduced by a dependent claim will not result in a claim from which it depends receiving an unintended construction.
Additionally, a patent agent would be well advised to consider structuring claim dependencies, whether through the use of multiple-dependent claims or otherwise, so as to avoid subjecting an independent claim to multiple attacks based on claim differentiation.
Turning to litigation, when instructing an expert on the principles of patent law to be taken into account when preparing his or her report on construction, the principle of claim differentiation would be a worthwhile inclusion. As seen from the case law discussed above, a trial judge might prefer the opinion of an expert who has taken this principle into account over the opinion of an expert who has failed to give it any consideration.