The following article appears in the journal JOM, 42 (5) (1990), p. 59. JOM is a publication of The Minerals, Metals & Materials Society

The Doctrine of Equivalents in Patent Infringement

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.

When the claims of a patent are read against an accused practice, they may be so close to identical that infringement is clear. Also, the accused practice may be so remote from the patent that there is no possibility of infringement. Very often, however, there are some differences, requiring further study. Sometimes, such differences are incorporated into a design after knowledge of a patent in an effort to avoid infringement. Then, the question presented is whether the design is sufficiently different from the patent to be held to be non- infringing. If the design is too close to the patent, it will infringe. If the design is remote enough, it will not infringe. The U.S. Supreme Court has stated: "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. Outright and forthright duplication is a dull and very rare form of infringement."

One reason that literal infringement of a patent is a dull form of infringement is that where the potential infringer knows of the patent and takes steps to avoid infringement by making changes from the exact thing which is disclosed and claimed in the patent. At the same time, the individual may copy as much of the patent as thought possible without becoming liable for infringement. When that happens, the issue raised is whether the accused structure or process is the "equivalent" of what is claimed in the patent. A similar problem can arise where a practice is adopted without knowledge of a patent, and the patent becomes known only after a business commitment has been made to follow the practice.

The rule of law for determining equivalency as laid down by the Supreme Court is quite simple: "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." Despite the seeming simplicity of this rule, its application to a particular case is often complex. Determination of equivalency frequently involves conflicting opinions of experts and disputes as to scientific or engineering facts. The issue is often resolved by the testimony of expert witnesses and the decision may rest on which of the experts is the more believable. Things which are equivalent for one purpose may not be for other purposes. In one well- known case, the patent included claims for welding rods having a flux containing a major proportion of "alkaline earth metal silicate." The accused welding rods included a flux composed principally of "manganese silicate," which is not an alkaline earth metal silicate. Nevertheless, the accused welding rods were held to infringe because of testimony that manganese and magnesium were similar in many of their reactions and that they served the same purposes in fluxes.

Most patents are not issued with the claims originally filed. The claims of most patent applications are rejected, and the claims are amended with the inclusion of more detailed and restrictive language. If an examiner rejects the claims as unpatentable over the prior art and the claims are amended to read more narrowly to avoid the prior art, the patentee is barred from asserting the claims in the broader sense. Since he or she gave up the broader construction to obtain allowance of the claims, the patentee is not permitted to assert that the broader construction is the equivalent of the claim which was finally allowed. The process of rejection and amendment followed by allowance is shown by correspondence in the Patent and Trademark Office file. That file history is contained in a heavy paper jacket known as a "file wrapper." As a result, the rejection of a claim followed by a narrowing and more limiting amendment is known as a "file wrapper estoppel."

When a question of equivalency is under study, it is also necessary to know if the patent is a pioneer in a whole new field or if the patent shows only a narrow improvement of a subject that is well known. A pioneer patent will be given a much broader range of equivalents than one for a narrow improvement to existing technology.

It is not enough to rule out literal infringement. It is also necessary to determine equivalency. Equivalency can only be resolved by a careful analysis of the file history of the patent application. Such analysis should always be made when an accused structure or process resembles in any way a claim of a patent.

It is also possible for a device to be so far changed in principle from a patented device that it performs the same or a similar function in a substantially different way, even though it falls within the literal words of the claim. This situation can occur when the accused device is so far removed from the invention as disclosed in the patent that it is considered in law to be a different thing entirely. Even if the claims literally read on the thing which is accused, the claims are limited by construction to cover the invention which was disclosed in the patent and to exclude a thing which is different from the disclosed invention. This result is sometimes known as the "Reverse Doctrine of Equivalents."

Possible infringement of a patent claim must never be taken lightly. Issues of direct infringement, equivalency, file wrapper estoppel, and limitation of the claims to an invention as disclosed all must be considered before concluding that infringement does or does not exist.