George Mason University Antonin Scalia Law School

Professor Daryl Lim Explores the Doctrine of Equivalents and Equitable Triggers

The following post comes from Yumi Oda, an LLM Candidate at Scalia Law and a Research Assistant at CPIP.

files labeled as "patents"By Yumi Oda

The term “claims” may not mean much to many, but it means the world to most patent practitioners. As Judge Giles Rich once observed, “[t]he name of the game is the claim.” Claims are the “metes and bounds” of a patent application, essentially defining what the inventor invented. In this sense, claims have a notice function to demarcate the scope of the claimed invention, which in turn allows subsequent inventors to “design around” the disclosed invention and come up with new inventions. This is exactly the type of incentive the U.S. Constitution envisioned “[t]o promote the Progress of . . . useful Arts.”

The interpretation of claims (i.e., claim construction) often becomes a central issue in patent litigation. Notably, patent protection is not limited to literal infringement, but may extend to “equivalents” of the patented invention. Rooted in equity, the doctrine of equivalents (DOE) asks whether the accused device contains elements “equivalent” to each claimed element of the patented invention. However, since the DOE affords courts flexibility in expanding claims and undermining the notice function of patent claims, practitioners and judges have long struggled with its inherent conflict.

This is the challenge tackled in a recent paper that was published in the Santa Clara High Technology Law Journal entitled Judging Equivalents by Professor Daryl Lim, a Professor of Law at John Marshall Law School. The paper, which comes from CPIP’s Thomas Edison Innovation Fellowship program, provides a contemporaneous, empirical survey of the law and literature on the DOE over the past 150 years in order to help contextualize its nature and evolution and to chart its future.

The Doctrine of Equivalents

Prof. Lim starts by looking at the origins and scope of the DOE. Claims were in fact not required in earlier patent legislation, so infringement at the time focused on the “essence” of the patented device through an inquiry into equivalence. This earlier tradition persisted even after the requirement for claims was established in 1836. However, with the introduction of “peripheral claims” (claims setting forth the exact boundary of the claimed invention) by 1870, the focus shifted to the literal language of the claims, with equivalents invoked only when required by equity.

Due to what Prof. Lim calls “ad hoc and amorphous boundaries,” as well as the fact-intensive inquiry involved, the DOE remains unruly and challenging to administer. Originally, the Supreme Court formally recognized the DOE in 1853, which may have been justified by the Lockean theory of justly rewarding inventors for their labor. However, in 1950, the Supreme Court shifted its focus and justified the DOE as an equitable safeguard for the defendant’s unjust enrichment. Nevertheless, the DOE was met with skepticism from its early days, and the creation of the Federal Circuit in 1982, although meant to resolve circuit splits, only added to the existing confusion. Moreover, when the Supreme Court set out a couple of limitations to the doctrine in 1997, questions regarding the applicability of these limitations further deepened the division.

Prof. Lim then moves on to describe the history and development of the doctrine in three words: incoherence, incompetence, and irrelevance.

To begin, “incoherence” refers to the courts’ “perennial struggle for clarity.” The Supreme Court has so far employed two alternative tests—“function-way-result” test and “insubstantial differences” test—without clarifying which test should be chosen. Additionally, the Federal Circuit has also failed to provide guidance regarding an appropriate formula, which in turn has forced lower courts to determine equivalents on an ad hoc basis. Prof. Lim partly attributes this “incoherence” to the inherently ambiguous nature of claim construction, which involves a question of law (what the claims mean to a hypothetical “person of ordinary skill in the art”) and a question of fact (whether the accused device falls within the scope of the claim, literally or under the DOE).

This ambiguity has long raised concern in the courts about the jury’s competence in comparing the construed claims to the accused device, which leads to the next issue of “incompetence.” The jurors’ expected incompetence in the face of the complexity of their given task, combined with their substitution bias generally favoring inventors and the Patent Office and procedural limitations resulting in the unreviewable status of their general verdicts, led to the conventional wisdom that judges, who are often “repeat players” in patent litigation, are better suited for this task. However, Prof. Lim’s data notably shows that the jury’s incompetence may in fact have little factual basis.

Finally, Prof. Lim describes a substantial decline in cases where the DOE applies. This “irrelevance” of the DOE now leads us to the question of what role equitable triggers play in this decline.

Equitable Triggers

Equitable triggers can affect how widely or narrowly courts apply the DOE. Surprisingly enough, most (i.e., over 70%) of the cases examined did not even mention equity in any form, despite the doctrine’s equitable origin, suggesting the equitable nature of the cases did not determine their outcomes.

1. Copying. Copying is permitted as an intermediate step of designing around the patent as long as the defendant can show his or her new device “does more than just narrowly escape the claim.” Overall, copying has not been a prominent type of trigger, but it played a significant role in rivalry. In fact, among different types of equitable triggers, patentees were most favored with copying.2. Design-Arounds and Independent Invention. Design-arounds and independent invention (rivals producing their devices without knowledge or notice of the patented device) are justified by the basis of leapfrogging. Consistent with the copying cases, non-rival defendants were significantly more likely to prevail compared to rival defendants.

3. Pioneer Inventions. In contrast, pioneer inventions are justified because of a wide gap between the claimed invention and the prior art. Prof. Lim concludes that pioneer inventions were extremely rare, and no meaningful conclusion was drawn here, indicating that there is little incentive for patentees to even assert pioneer status.

In addition to these equitable triggers, Prof. Lim carefully draws a distinction between the DOE and means-plus-function (MPF), which is another common deviation from the literal meaning of the claims. MPF under 35 U.S.C. § 112(f) allows the scope of claims to be expanded from their literal meaning to capture similar structures described in the specification, performing the same function as the structure recited in the claim, and known at the time of patent filing. In contrast, the DOE expands beyond what has been described in the specification, and limits equivalence to newer technologies developed after the patent grant.


Subsequently, Prof. Lim looks at four court-established bars to the DOE, each penalizing patentees “for sloppy or overly aggressive patent drafting and for strategic behaviors that shift the cost of information . . . from an inventor to the Patent Office and the public.” In practice, these limitations allow judges to absolve defendants from baseless patent infringement claims, as shown by summary judgment being the dominant procedural posture.

1. Prosecution History Estoppel. Prosecution history estoppel (PHE) prevents patentees who made a certain type of narrowing amendment from exploiting the DOE and asserting a contradictory, broadening scope of the claims later. Overall, defendants were on the winning side across the almost entire 10-year period in cases where PHE was employed, confirming the conventional wisdom that patentees generally do worse than defendants.2. “All-Elements” Rule. The “all-elements” rule requires the DOE to be applied to individual limitations of the claim, rather than to the invention as a whole. While patentees generally did worse than defendants in these cases, patentees did significantly better in 2009-2018 compared to earlier years.

3. Prior Art Bar. The prior art bar limits patentees to a scope that avoids prior art. Patentees won by a slight margin here—by far the best among the four limitations to the doctrine—because of the patentees’ expertise to navigate prior art arguments and survive summary judgment motions filed by defendants.

4. Public Dedication Rule. The public dedication rule assumes that, when a patentee discloses but fails to claim subject matter, the unclaimed subject matter is dedicated to the public. This is to prevent patentees from filing broad disclosure, while presenting only narrow claims (to avoid the claims being examined in view of more prior art), and then exploiting the DOE later only to capture the broad disclosure. While this rule continues to play a relatively small role, patentees did better more recently, as with PHE and the “all-elements” rule.

In conclusion, Prof. Lim’s paper offers a contemporaneous, doctrinal, and empirical survey of the doctrine of equivalents. And as the first empirical study on “equitable triggers,” it can fill a significant gap in the literature and help evidence-based decisionmaking in patent law and policy.