Chapters 4 & 5: Claim Construction & Infringement copyright 1996 - 1999 Donald M. Cameron, Aird & Berlis Cameron's Canadian Patent & Trade Secrets Law:Home Page; Index Chap. 1, Chap. 2, Chapt. 3, ..., Chap. 5, Chap. 6, Chap. 7, Chap. 8, Chap. 9, Chap. 10, Chap. 11

A Precise Test for Claim Construction

by Donald M. Cameron

Summary

Scientific Verity

Before we look at what Mr. Armstrong described as "a world where men substitute words for reality and then talk about the words", we should first examine Armstrong's world of science and engineering, where people "deal with realities and where truth is, in fact, the goal." Until it is understood what scientists and engineers are dealing with, our words describing their reality will not do the job adequately.

Firstly, science is not nature; science is our description of nature. It is sometimes only an approximate description. Dr. Richard Feynman, a brilliant physicist and one of its best teachers conceded that

"...we do not yet know all the basic laws: there is an expanding frontier of ignorance... Each piece, or part, of the whole of nature is always merely an approximation to the complete truth, or the complete truth so far as we know it. In fact, everything we know is only some kind of approximation, because we know that we do not know all the laws as yet. Therefore, things must be learned only to be unlearned again or, more likely, to be corrected.

The principle of science, the definition, almost, is the following: The test of all knowledge is experiment. Experiment is the sole judge of scientific "truth." But what is the source of knowledge? Where do the laws that are to be tested come from? Experiment, itself, helps to produce these laws, in the sense that it gives us hints. But also needed is imagination to create from these hints the great generalizations--to guess at the wonderful, simple, but very strange patterns beneath them all, and then to experiment to check again whether we have made the right guess."

It is the recognition of patterns in nature that allow scientist to create the so-called "laws" of nature. These are in fact laws of humans, and are subject to being overthrown by the "more correct" laws of nature themselves through experimentation..

So whether scientists use words or mathematical formulae to describe natural "laws", these "laws" all are approximations, imperfect abstractions based on our imperfect observations and incomplete understanding.

In using numbers to describe scientific phenomenon, scientists have recognized this uncertainty in the way that they express the observed quantity. If there are 52 grams measured in one sample and 55 grams measured in a second sample, the total would be expressed as 110 grams plus or minus 5 grams because there are only two "significant digits" in the initial samples. A scientist would read this as meaning the total is somewhere between 105 and 115 grams, but the inherent errors in the measurements do not give us a clearer idea of the exact value of the total.

Numbers are rounded upwards from the half below and downwards up to the half above. For example something having a value of 6 could be any value between 5.5 and 6.49. On the other hand when scientists describe something as having a weight of 6.002 grams, they mean that it is between 6.0015 and 6.00249. Thus the number of significant digits expressed is an indication of the precision of the value - the specificity of the description.

Precision and Accuracy

For the purposes of this paper, I will use terms "precision" and "accuracy" as they are commonly used by engineers:

"Precision" defines the error, scatter or degree of uncertainty surrounding a fact

"Accuracy" is a measure of correctness or "truth".

We deal with precision and accuracy in our everyday lives.

Something can be precise without being accurate. Our quartz wrist watches can tell us the time within seconds and can maintain that precision for months or years but are only accurate if we calibrate them by setting them according to "the correct time". Our bathroom scales are precise to within two pounds (the gradations on the scale) but are not necessarily accurate (Compare the measurement at home with that at the doctor's office!).

In the bulls eye shown below, the grouping of the holes is an indication of the precision that was achieved by the shooter. The holes are generally grouped close together showing good precision, but are offset from the bullseye a noticeable distance to the right, showing less than perfect accuracy.

Likewise, we can have accuracy with varying degrees of precision. All of these answers to the question "How old are you?" are accurate, depending upon the degree of precision wanted by the questioner:

• 30 years
• 30 years, one month
• 30 years, 36 days, 12 hours and 15 minutes.

What is Claim Construction?

A patent claim is a single sentence describing the invention as claimed by the inventor(s). The inventor creates an idea and a way of carrying it out: an embodiment of the invention. The patent claim is a higher-level functional or generic description of the embodiment of the invention. The claim is not meant to protect a specific embodiment of the invention - instead, it is meant to protect the invention incorporated in the embodiment. The claim should contain the minimum number of words necessary to protect the invention - anything more is unnecessary verbiage.

How then showed a court construe a patent? Should it:

• limit the inventor's monopoly to the exact wording of the claims, thereby giving others a clear direction as to what can or cannot be used, or
  
• protect something more than what the literal language of the claim covers, for example, what the inventor could have and perhaps should have claimed, so as to prevent a competitor from taking advantage of the invention by making minor changes from the exact language of the claims?

Currently, the Canadian Courts do both, or either depending upon the judge and the particular facts of the case.

The Canadian Courts are using two mutually inconsistent tests:

• one of strict construction where the words of the claim are limited to their plain meaning and none can be ignored or altered;

• the other, amending the language of the claim to broaden its scope beyond that of the strict language (referred to in the decisions as infringement of the "pith and substance" or infringement by the "doctrine of equivalents", referred to in this paper as "equitable infringement").

All the while, Canadian judges and their English counterparts state that they are consistently applying only one rule of construction: a purposive construction.

The use of these two very different tests results in unpredictable and inconsistent results and uncertainty and dissatisfaction amongst litigants.

The Patent Claim

Section 27(4) of the Patent Act⁽²⁾ requires the patentee to end the patent with the claims:

"The specification must end with a claim or claims defining distinctly and in explicit terms the subject-matter of the invention for which an exclusive privilege or property is claimed."

Interestingly, the statutory requirement that patent end with claims was introduced to the Canadian Patent Act by 1923 (s.14, The Patent Act, S.C. 1923, c.23).

The patent's claims define the monopoly in words. The ambit of the invention must be circumscribed by definite claims.⁽³⁾ A patent may have many claims, each defining the invention in different words and describing it broad or narrow functional language.

In Canada, claims are most commonly of the form having a brief preamble describing the apparatus or method in terms of its field of use, the transition "comprising"^(3a) , and followed by the claim elements (apparatus elements or process steps) which are like a checklist of the components of the claimed invention. Crudely speaking, if an potential infringer's device or process has each and every element listed in a claim, then there is infringement of that claim.

Claims are sometimes analogized as a series of "fences" surrounding and protecting the valuable invention. The claims define the metes and bounds surrounding the protected invention, much like surveying terminology defines the metes and bounds to a piece of land containing gold. The fences must be clearly defined in order to give the necessary warning. Property which is not owned by the inventor must not be fenced in, otherwise the claim will be invalid as being broader than the invention.⁽⁴⁾

Claims are usually drafted with multiple, dependant claims. As such, they are like a set of nested fences, each fence surrounding a more particularized form of the invention. If one of the outer most fences fails because it is invalid, the inner ones remain to protect the invention. The larger fences are the broad claims; the narrowest fences are the narrow claims.

Another way of considering a claim is to describe them by use of a Venn diagram, a mathematical illustration used to identify "sets". The Venn diagram of some claims would be analogous to an aerial photograph of the "fences" surrounding the invention. The claims would be a series of sets and subsets, all including the invention at the centre of the sets. Anything that met the description of that set, would fall inside that claim and infringe it.

The invention can be described in a number of ways and therefore, there can be different sets of claims each with dependant claims, all protecting the invention at the core.

Claim Drafting

While it is beyond the scope of this paper to explain how one drafts a patent claim, the intention of the patent drafter must be appreciated so that its general rules of construction can be appreciated.

Because the claim is supposed to protect the functionality of the invention and not be limited to the specific "nuts and bolts" that inventor may have built, the patent drafter attempts to create a "higher level" description of the invention, extracting only what is essential to the operation of the invention. Thus an inventor should only claim what is considered to be essential to the use of the invention.

In this respect, the role of the patent drafter is much like that of a scientist trying to create a general theory as to how the device or process works. If the inventor has designed or built one of a species, the patent drafter attempts to claim the genus or family.

For example, if the inventor has produced a device that has two parts ("members") held together by nuts and bolts, the patent drafter (with the aid of cross-examining the inventor as to what is needed and what is not) would soon appreciate that the same function could be served by a rivet or by a nail. The patent drafter might combine these particular devices into a more generic descriptor, such as "fasteners" or "fastening means", or better still might recognize that the members need only be connected (by fasteners or by being molded together as an integral piece) and thus might settle on language such as "the first member being connected to the second member" without narrowing the claim to be limited to devices held together by fasteners. This would include all manners of nuts an bolts, rivets, nails and other fasteners or even members that are integrally connected (ie. unitary construction). By describing the invention in higher and higher levels of functional abstraction, the claim (and the resulting protection) becomes broader. The patent drafter should also include claims citing specific hardware elements in case the broader claims are later found to be too broad as encompassing later revealed prior art

The choice of the wording used to describe the invention is that of the inventor (or, more accurately, the patent drafter). The inventor may be as cautious and as bold as he or she desires, but once the language of the claims is finalized during the prosecution of the application, they cannot be changed (subject to reissuance or reexamination procedures).

Because the claim is a checklist, the patent drafter tries to use as few words as possible: the greater the number of words, the easier it will be for someone to avoid infringement by eliminating claimed elements.

The Role of the Court in Claim Construction

There a some general rules of claim construction that appear to have survived even the most illogical of decisions. They are set out below.

The First Duty

The first duty of the Court is to construe the claims alleged to be infringed⁽⁵⁾ to give them meaning and to determine the scope of the claims.⁽⁶⁾ This is to be done before issues or infringement⁽⁷⁾ or validity⁽⁸⁾ are determined so that a consistent reading of the patent is applied to questions of infringement and validity.

A Question of Law

Claim construction is a question of law.⁽⁹⁾ A Canadian patent trial judge hears all the evidence at trial and determines construction after hearing final argument making the ruling in the Reasons for Judgment.

Because many U.S. patent trials are decided by judges and juries, American patent trials since 1996 have held Markman hearings (named after the case when the practice began⁽¹⁰⁾). Before the trial by jury begins, the judge hears evidence and construes the patent. The jury then deals with the issues of fact such as infringement.

Pre construction "set up"

The judge must put him or herself in the proper mindset to read the patent:

(1) expertise

The patent should be read and understood from the viewpoint of the addressee of the patent: a skilled worker in the field of the invention of the patent.⁽¹¹⁾ In some cases, the addressee may be a team of people having different expertise.⁽¹²⁾ Only those familiar with the field of the invention can be expected to make anything of the claims.⁽¹³⁾ The knowledge that such a person is expected to possess is to be taken into account.⁽¹⁴⁾ The question to be asked by the Court is: How would a workman in the trade understand it?⁽¹⁵⁾

The claims are to be interpreted by applying the common vocabulary of the art.⁽¹⁶⁾ For example: if a claim used the word "bolt", what would that term mean in the context of that patent? Depending upon the art to which the patent is directed, it could mean a threaded screw to be used in conjunction with a nut as a fastener, an 8 foot length of a tree trunk or a length of cloth wrapped around a cardboard spindle. It is language as generally understood, not that defined by an obscure reference.

Although the interpretation of the claims is a matter for the Court, the Judge may be assisted by the evidence of an expert who is knowledgeable in the field to which the claim is directed. Such evidence may relate to the state of the art at the date of the patent, the meaning of technical terms and the working of the invention.⁽¹⁷⁾ In the past, an expert witness was not entitled to state what the specifications meant nor whether any given step or alteration was obvious.⁽¹⁸⁾ This has given way to the current practice of expert witness giving opinion on virtually every issue, including infringement or validity, and the Judge applying whatever weight should be given to such evidence.⁽¹⁹⁾

(2) interpretation as of what date

Although the debate continues as to the effective date on which a patent is to be interpreted (the date of the filing of the first application, or the date of issuance of the patent), current authorities state that a patent is to be construed as at the date it issued.⁽²⁰⁾ This factor is only relevant if the meaning of the words used in the patent have changed over time.

(3) attitude of the reader (Court)

The patent should be read by a mind willing to understand, not a mind desirous of misunderstanding. It ought not to be construed malevolently, but need not be construed benevolently.⁽²¹⁾ It must be fairly and reasonably construed, being fair to both the patentee and the public.⁽²²⁾ The patent should be read to afford protection to what has been invented.⁽²³⁾ A patent should be approached with a judicial anxiety to support a really useful invention. One should not be too astute or technical in raising objections to the specifications.⁽²⁴⁾

(5) objectiveness

Some cases say that the construction of the claim is to be done without having in mind the alleged infringement⁽²⁵⁾ or as one judge described it, "as if the defendant had never been born" (Judge Ford, UK Patents County Court).

Although objectivity is a laudable principle, in fact there is usually only a word or phrase or two in the entire claim that is at issue in the lawsuit and it makes sense that the Court need only focus on that part of the claim at issue when it construes the language. Although the structure of the defendant's allegedly infringing device is irrelevant to issues of construction, the Court must at least be aware of what elements of the claim is alleged by the defendant not to be present in the defendant's device.

Construing the patent

It has been said that the Court should construe a patent just as it would construe any other document such as a written contract.⁽²⁶⁾

The process by which patents are issued suggests that the concept of contra preferentem be borrowed from the principles of contract interpretation: the terms of the contract will be interpreted against the person who drafted it. Thus if an inventor decides to include a certain element in his or her claim, and patentees are supposed to only claim what is essential to the working of their invention, why should the Court ignore that element in construing that claim?⁽²⁷⁾

Step #1 The patent should be read as a whole

The document should be construed (and read) as a whole.⁽²⁸⁾ During the reading, the disclosure should be examined for definitions which state the meaning of words used in the patent. Such definitions apply to the words used in the claims, thereby defining their scope.⁽²⁹⁾ Unless a term has been defined in the dictionary sense in the disclosure, or by reference to a publicly available document,⁽³⁰⁾ a word or phrase should be given its ordinary meaning.⁽³¹⁾ The words should be given a natural and not a strained meaning.⁽³²⁾

Rule #1: Don't make reference to the disclosure unless the words in the claim are ambiguous

Reference may be made to the disclosure "... in order to understand what the former says" but only where the words of the claim are ambiguous. Reference to the disclosure is improper to vary the scope or ambit of the claims.⁽³³⁾

The specification is not the dictionary by which the scope and effect of the terms of the claim is to be ascertained, in particular where the claim is expressed in simple and direct language or in wide or general terms whose meaning is plain and unequivocal.⁽³⁴⁾

Rule #2 The disclosure cannot vary the scope or ambit of the claims if the language of the claim is plain and unambiguous.

A patentee cannot use general language in the claim and subsequently restrict or expand it to qualify what is expressed in the claim by borrowing from other parts of the patent.⁽³⁵⁾

You cannot diminish the ambit of the monopoly claimed merely because in the disclosure, the patentee has described the invention in more restricted terms than in the claim itself.⁽³⁶⁾ The invalidity of a claim cannot be avoided by attempting to narrow the scope of a claim based on language used in the disclosure. To do so would be like throwing scientific dust in the eyes of the Court.⁽³⁷⁾ The reason for this is that patent drafters might make the claim very wide upon one interpretation, in order to prevent as many people as possible from competing with the patentee's business, and then try to rely on carefully prepared sentences within the disclosure to limit the claim so as to be valid.⁽³⁸⁾

Fox suggested that variations described in the disclosure may have the effect of widening the scope of the claims by showing that there may a substitution of means.⁽³⁹⁾

The claims are not limited to the specific examples or embodiments described in the patent.⁽⁴⁰⁾ The Court cannot limit the claims by simply saying that the inventor must have meant to claim what he had described in the disclosure. If the claims are broader than the disclosure and go beyond the invention, the patent will be declared invalid.⁽⁴¹⁾

If the disclosure states in the "Summary of the invention" section that there are certain "objects" to the invention, you cannot read those objects into a claim when interpreting the claim.⁽⁴²⁾

Use of external material to interpret a patent

Parole evidence is admissible to explain the meanings of words or technical matters and to inform the Court of relevant surrounding circumstances.⁽⁴³⁾

Notes made by an inventor are not relevant in construing the claims,⁽⁴⁴⁾ nor is the testimony of the inventor.⁽⁴⁵⁾ The inventor is not the best witness to claim construction. The inventor will have a strong self-interest and may not be reading the claim as an objective and first-time reader.⁽⁴⁶⁾

The patent should not be limited in scope by prior art.⁽⁴⁷⁾

use of the file wrapper

The "file wrapper" is the name given to the file in the Patent Office containing the correspondence between the inventor's patent agent and the patent office examiner during the prosecution of the patent. It sometimes contains statements made on behalf of the inventor as to what is the invention or how it differs from the prior art.

The Canadian or foreign patent application file relating to the patented issue cannot be admitted into evidence in Canada to limit the construction put on the claims.⁽⁴⁸⁾

The file wrappers have been ruled inadmissible when they relate to changes made during the application⁽⁴⁹⁾ or to stop a party from denying that certain elements were "essential elements" to a claim (Amfac at p. 86-87).⁽⁵⁰⁾

The absolute exclusion of the admissibility of file wrappers may be weakening (See AlliedSignal trial reasons at p. 31)

claim differentiation

Each claim is to be construed and given a distinct meaning. Differently worded claims are to be interpreted differently.

Strict vs. Equitable Construction

The Courts of "Law" (as compared to the Courts of Equity) limited the relief they would grant. In the case of contractual relief, parties were often required to comply with the strict wording of their agreement. On the other hand, the Courts of Equity would strive to do what was fair in the circumstances, even if it required departing from the strict wording of a contract.

This split personality is evident in the strict construction and equitable construction techniques described below.

Strict Construction

If the words of the claim, upon a plain reading of them, aptly describe the defendant's device or process, then there is said to be "literal infringement". This result seems self evident and straightforward - so much so that very few cases are litigated all the way to trial because the outcome is so predictable.

Under strict construction of a claim, all elements are considered to be essential to the invention). The absence of any claimed element, under the method of strict construction results in a finding of non-infringement.

One of the earliest, and oft cited case is Electric and Musical Industries, Ltd. et al v. Lissen, Ltd. et al⁽⁵¹⁾ where Lord Russell of Killowen held at p. 39:

"The function of the claims is to define clearly and with precision the monopoly claimed, so that others may know the exact boundaries of the area within which they will be trespassers. Their primary object is to limit and not to extend the monopoly. What is not claimed is disclaimed. The claims must undoubtedly be read as part of the entire document, and not as a separate document; but the forbidden field must be found in the language of the claims and not elsewhere. It is not permissible, in my opinion, by reference to some language used in the earlier part of the specification to change a claim which by its own language is a claim for one subject-matter into a claim for another and a different subject-matter, which is what you do when you alter the boundaries of the forbidden territory. A patentee who describes an invention in the body of a specification obtains no monopoly unless it is claimed in the claims. As Lord Cairns said, there is no such thing as infringement of the equity of a patent (Dudgeon v. Thomson, L.R. 3 App. Cas. 34)."

The most recent example of strict construction from the highest authority (the Federal Court of Appeal) was O'Hara Manufacturing Ltd. et al v. Eli Lilly & Co. et al ⁽⁵²⁾

The patent claim related to a device for coating tablets that resembled a coin-operated clothes drier. As shown in the patent drawing, Figure 2 below, the tablets tumbled as a bed 17. Air was drawn through the bed of tablets by means of a vacuum system drawing air through plenum 21 that road on wheels 25 along the foraminous (provided with holes) outer surface of the drum. Because the drum 11 was not perfectly circular, the plenum was pressed against the drum to have its wheels ride against it, or in the words of the claim, the plenum was "flexibly biased against the drum" by piston 34 and levers 32 and 29.

Thomas Engineering, Eli Lilly's licensee, learned to make the drum more truly circular and did away with the piston and lever arm, fixedly securing the plenum on the frame so that it was spaced slightly away from the drum, touching it only with a resilient (flexible?) skirt. O'Hara copied the improved commercialized design without the piston arrangement.

At p. 5

"According to that theory, a court in deciding whether there is infringement, must first interpret the claims and determine whether there is textual infringement, and if it does not find textual infringement, it must engage in the difficult process of distilling the claims and extracting the substance of the invention so as to determine whether the defendant did steal it."

At. p. 5-6

"... the patentee is tied strictly to the invention which he claims and the mode of efffecting an improvement which he says is his invention."

The Court of Appeal held, that because O'Hara lacked the feature of "flexibly biasing the plenum against the drum", there was no infringement. More importantly, the Court said that it could not rewrite the patent claim to omit the feature that was later discovered to be unnecessary.

"A court must interpret the claims; it cannot redraft them. When an inventor has clearly stated in the claims that he considered a requirement as essential to his invention, a court cannot decide otherwise for the sole reason that he was mistaken." ⁽⁵³⁾

Equitable Infringement

Equitable infringement is invoked when the claim is narrower than the invention that could have been claimed, validly, by the inventor. The court exercises its equitable jurisdiction to "stretch" the exact language of the claim to either ignore the unnecessary claim element or to substitute equivalents for the elements.

For example, imagine a claim reciting four elements:

• body means adapted to support a user;
• wind propulsion means;
• a universal joint connecting said body means to said wind propulsion means and;
• a green pennant attached to said wind propulsion means
  

It is clear that the fourth element is unnecessary for the proper functioning of the invention. The patent drafter should never have included the element in the claim. Who is to pay for that mistake?:

• the inventor, by having the monopoly limited to sailboards having green pennants?

• the infringer, by having the courts ignore that limitation and find there to be infringement even if no green pennant is present? or

• the patent agent who perhaps negligently included an unnecessary element?

In effect, under the equitable infringement cases, the Court rewrites the claim, to a more abstract description of the invention to a higher level of abstraction than was chosen by the inventor or the patent agent when the patent was sought. The element is either eliminated altogether or a functional equivalent is substituted.

In Canada, the Courts do not admit that they are rewriting the claims. Like the Emperor with new clothes, they appear to be the last to recognize (or admit) what is happening.

Pith and substance

The leading case in Canada for infringement of the "pith and substance" of a patent is McPhar v. Sharpe⁽⁵⁴⁾.

In the McPhar case, the claims called for "... means to suspend said transmitting coil to hang freely in a vertical plane but orientable in azimuth..." and "... a transmitting coil suspended to hang vertically and orientable in azimuth".

The defendant mounted its coil on top of a tripod, allowing free rotation about the azimuth (vertical axis).

The Court held that there was infringement under the "pith and substance" doctrine.

Under the "pith and substance" doctrine, the claim is analyzed and divided into two sets of elements: those that are deemed "essential" and those that are deemed "non-essential". Variations or omissions of non-essential elements do not change the way the invention works. One is considered to have taken the substance of the invention if one uses all the essential elements; non-essential elements can be omitted or varied.

Since form follows function, any change to the elements of the patent will, in some way, change how the invention works - unless the element is as frivolous as the green pennant example given above. A more proper question to ask is whether the change to the way the invention works is, for all practical purposes, negligible.

The doctrine of "pith and substance" flies in the face of the case law that says that the Court can only interpret, and must not rewrite the claims.⁽⁵⁵⁾ Likewise, it prevents there being a clear "fence" surrounding the invention (one of the purposes of claims) so that others can avoid infringement. The doctrine of "pith and substance" has created the most uncertainty as to the scope of a patent (and thus spawned considerable litigation).

The leading cases dealing with "pith and substance" include:

• Clark v. Adie (1876-7), 2 App. Cas. 315, where the Lord Chancellor (Lord Cairns) held at page 320 that non-essential parts may be omitted:

"The infringer might not take the whole of the instrument here described, but he might take a certain number of parts of the instrument described; he might take an instrument which in many respects would resemble the patent instrument, but would not resemble it in all its parts. And there the question would be, ..., whether that which was done by the alleged infringer amounted to a colourable departure from the instrument patented, and whether in what he had done he had not really taken and adopted the substance of the instrument patented. And it might well be, that if the instrument patented consisted of twelve different steps,..., an infringer who took eight or nine or ten of those steps might be held by the tribunal judging of the patent to have taken in substance the pith and marrow of the invention, although there were one, two, three, four or five steps which he might not actually have taken and represented upon his machine."

• and in R.C.A. Photophone Ld. v. Gaumont-British Picture Corporation Ltd. and British Acoustic Films, Ld. (1936), 53 R.P.C. 167 at 197, Romer L.J. held that non-essential elements could be substituted with mechanical equivalents:

"The principle is, indeed, no more than a particular application of the more general principle that a person who takes what in the familiar, though oddly mixed metaphor is called the pith and marrow of the invention is an infringer. If he takes the pith and marrow of the invention he commits an infringement even though he omits an unessential part. So, too, he commits an infringement if, instead of omitting an unessential part, he substitutes for that part a mechanical equivalent."

It should be noted that while the inclusion of claims became compulsory in the U.K. in 1883, the specification in Adie ended with a single general claim. The specification in RCA ended with a full list of claims. As mentioned above, the statutory requirement that patents end with claims was introduced to the Canadian Patent Act by 1923 (s.14, The Patent Act, S.C. 1923, c.23).

The doctrine of equivalents therefore allows one to claim one member of a species (which species contains equivalents of what has been claimed) but obtain protection for the next higher order - the genus.

For example, one could claim, as a claim element, a bolt. Mechanical equivalents would include a nail and a rivet. These fasteners would be part of the genus "fastener". Thus the Court rewrites the claim to one level of abstraction higher than that contemplated by the patent drafter.

Catnic's "purposive construction"

In the Catnic⁽⁵⁶⁾ case, the Catnic patent called for a lintel (the bar that goes across the top of a window to support the weight of the bricks laid above it) to have a rear wall member which should "extending vertically". A cross-sectional view of the Catnic lintel from the Catnic patent drawings is shown in Figure 1 to the left. The rear wall is highlighted in yellow.

Hill copied the Catnic design, then after receiving a writ from Catnic, changed the design so that the rear member was inclined 6 or 8 degrees from the vertical. This resulted in the Hill lintels being 0.6 or 1.2 per cent weaker in their load bearing capacities. Functionally, this was a negligible difference, rendering the lintel only slightly weaker than the one shown in the patent. The Hill lintel is shown in Figure 2, with the inclined rear wall contained in the yellow highlighting.

The Trial Judge, Whitford J., held that there was no literal infringement but there was infringement under the "pith and marrow" doctrine. The Court of Appeal held that the limitation that the rear member should "extend vertically" was an essential feature and there could be no infringement of the "pith and marrow".

The House of Lords attempted to end the "pith and substance" test and replace it with new simpler test: "purposive construction".

1. Does the element at issue affect the way the invention works?

2. If it obviously does not, is it clear from a reading of the patent that the inventor did not intend to limit the invention to exclude a variant of the element?

If the second question is answered in the affirmative, then the variant is permitted and the infringement is caught.  The second part of the test is a difficult one to apply since its formulation combines a negative with an intention to exclude. The patent drafter would be trying to draft what was intended to be included rather than what to exclude. All non-included matters might not have been thought of at all.^(56a)