A sword available to the patentee—and a shield for the alleged infringer.

The patent laws “promote the Progress of Science and Useful Arts” by rewarding innovation with a temporary monopoly. U.S. Const. art. I, § 8, cl. 8. That monopoly is a property right, and like any property right, it should have clear boundaries. The clarity is essential to promote progress because it enables efficient investment in innovation. A patent owner should know what he or she owns, and the public should know what the patent owner does not own. For these reasons, the patent laws require inventors to describe their work in “full, clear, concise, and exact terms” (35 U.S.C. §112) “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 investor’s exclusive rights.” Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., 535 U.S. 722, 731 740, 122 S. Ct. 1831, 1837, 152 L. Ed. 2d 944, 955 (2002) (citing Bonito Boats, Inc. v. Thunder Craft Boats, Inc., 489 U.S. 141, 150, 103 L. Ed. 2d 118, 109 S. Ct. 971 (1989)).

In determining whether an accused device, process, 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. Graver Tank & Mfg. Co. v. Linde Air Prods. Co., 339 U.S. 605, 607, 70 S. Ct. 854, 94 L. Ed. 1097 (1950). Under the doctrine of equivalents, however, a product or process that does not literally infringe on the express terms of a patent claim may still be found to infringe if there is “equivalence” between the elements of the accused product or process and the claimed elements of the patented invention. Id. at 609. Although contrary to the general principle that the express and exact claims of the patent measure the scope of the patent monopoly, the doctrine of equivalents was developed to prevent persons from practicing frauds on patents and otherwise to ensure a fair scope of protection for patent owners. See id.

Over 150 years ago, in 1854, the Supreme Court observed that “[t]he reason why [the patent in question] covers only one geometric form, is not that the patentee has described and claimed that form only; it is because that form is capable of embodying his invention; and, consequently, if the form is not copied, the invention is not used.” Winans v. Denmead, 56 U.S. 330, 343, 14 L. Ed. 717, 722 (1854).

Where form and substance are inseparable, it is enough to look at the form only. Where they are separable; where the whole substance of the invention may be copied in a different form, it is the duty of the courts and juries to look through the form for the substance of the invention—for that which entitled the inventor to his patent, and which the patent was designed to secure; where that is found, there is an infringement; and it is not a defence, that it is embodied in a form not described, and in terms not claimed by the patentee.

Id. at 343.

The Supreme Court in Winans—for what appears to be the first time—explicitly recognized the concept of equivalent structure and form in comparing an allegedly infringing item with the patent in suit. The scope of a patent is not limited to its literal terms; instead, it embraces all equivalents to the claims described. Id.

As explained by Justice Jackson in Graver Tank & Manufacturing Co. v. Linde Air Products Co. one hundred years after Winans, courts have recognized that

to permit imitation of a patented invention which does not copy every literal detail would 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. Outright and forthright duplication is a dull and very rare type of infringement. To prohibit no other would place the inventor at the mercy of verbalism, and would be subordinating substance to form. It would deprive him of the benefit of his invention and would foster concealment, rather than disclosure, of inventions, which is one of the primary purposes of the patent system.

Graver Tank, 339 U.S. at 607, 70 S. Ct. at 856, 94 L. Ed. at1102.

The Graver Tank Court goes as far as to equate the use of the doctrine of equivalents with the underpinnings of the U.S. patent system—disclosure. The governmental function in a patent system is to provide an incentive for inventors to use the system and thereby make disclosure that others presumably will look at and build on by filing their own patents. Justice Jackson in Graver Tank observed that failing to implement the doctrine of equivalents would undermine this governmental function because it would create a disincentive for inventors to file patents because the patents would be so easy to design around to avoid infringement as to make them useless. Id.

The other side of this argument is that employing the doctrine of equivalents and extending the monopoly of the patent beyond the literal words in the patent claims can create substantial uncertainty about where the patent monopoly ends. Warner-Jenkins Co. v. Hilton Davis Chem. Co., 520 U.S. 17, 29, 117 S. Ct. 1040, 1049 137 L. Ed. 2d 146, 161 (1997). To reduce this uncertainty, the Werner-Jenkins Court held that when a patentee responds to a patent office rejection during patent prosecution by narrowing the claims in the patent—effectively, taking out claim scope to get around prior art—this prosecution history estops the inventor from later arguing that the subject matter covered by the original patent application claims (now partially abandoned during prosecution) was nothing more than an equivalent in a later infringement suit. Id. A patentee’s decision to narrow his or her claims through amendment may be presumed to be a general disclaimer of the territory between the original claim and the amended claim. Festo Corp., 535 U.S. at 740, 122 S. Ct. at11842, 152 L. Ed. 2d at 960.

In a somewhat recent application of the doctrine of equivalents, the Court of Appeals for the Federal Circuit reversed the District Court for the Northern District of California’s finding of noninfringement under the doctrine of equivalents and reversed the grant of summary judgment. Brilliant Instruments, Inc. v. Guidetech, LLC, 707 F.3d 1342, 1348 (Fed. Cir. 2013). In this setting, the technology was directed to electrical circuits that measured the timing errors of digital signals in high-speed microprocessors. The Brilliant Instruments court found that although the capacitor in the accused device was not located in exactly the same place as the claimed capacitor (where it was in parallel with the shunt), the issue of whether the change in location was an insubstantial difference was a genuine issue of material fact that precluded summary judgment. Id. In so doing, the court observed that to succeed on an infringement allegation under the doctrine of equivalents, the patentee must demonstrate equivalence of the allegedly infringing device or process under the “function-way-result” or “insubstantial differences” tests. Id. at 1347.

The doctrine of equivalents is a sword available to the patentee, allowing an infringement claim against an alleged infringer who has made superficial changes to an item protected by a patent merely to get around infringement. The doctrine of equivalents also provides a shield to the alleged infringer who can demonstrate that the same so-called “superficial changes” that the patentee claims constitute the “equivalent” of the patented item were, in fact, part of the initial patent application that were voluntarily dropped by the patentee during patent prosecution and, therefore, the equitable doctrine of estoppel prohibits the use of the doctrine of equivalents as to those dropped items in the claims. In the patent world, this estoppel is called “file wrapper estoppel,” referring to the patent application prosecution history file.

As with transaction documents, wills, trusts, shareholder agreements, and all other documents that often wind up in litigation, the success of that litigation is—in large part—owed to the foresight and skill of those who prepared the documents. This is even more so the case in a patent setting, given the examination process that takes place for a patent application to mature into a patent and the limitations of language in explaining the technology disclosed in a patent. As pointed out by one court,

[a]n invention exists most importantly as a tangible structure or a series of drawings. A verbal portrayal is usually an afterthought written to satisfy the requirements of patent law. This conversion of machine to words allows for unintended idea gaps which cannot be satisfactorily filled. Often the invention is novel, and words do not exist to describe it. The dictionary does not always keep abreast of the inventor. It cannot. Things are not made for the sake of words, but words for things.

Autogiro Co. of Am v. United States, 384 F.2d 391, 397 (Ct. Cl. 1967).