Petitioners and patent owners in proceedings before the Patent Trials and Appeals Board (“PTAB”) should take note of several issues arising from the Federal Circuit’s recent decision in Williamson v. Citrix Online LLC.1 After theWilliamson decision, more claim terms may be construed as “means-plus-function” terms, even if they do not actually recite the word “means.” Expert testimony and other extrinsic evidence may be used to determine whether means-plus-function construction should apply. A given term, its prefix, and any inputs or outputs of the recited term are all factors that may be considered. Moreover, structure corresponding to the term should be identified in claim construction before the PTAB. Furthermore, allegations of indefiniteness will likely increase.

A means-plus-function limitation is a limitation of an apparatus claim that is recited purely in terms of its function and is governed by 35 U.S.C. § 112(f). The name derives from the common practice of drafting such limitations by reciting “means for” followed by a recitation of function. The means-plus-function limitation is narrowly construed “to cover the corresponding structure, material, or acts described in the specification and equivalents thereof.”2 Conventionally, claims including means-plus-function limitations are narrower than other claims and are more likely to be valid over prior art while less likely to be infringed.

The popularity of means-plus-function limitations among patentees has been plummeting in recent years.3 Nevertheless, Williamson may cause many limitations, originally intended not as means-plus-function, to be interpreted as means-plus-function limitations during review in front of the PTAB.

The Williamson Decision

Before the Williamson decision, a “strong” presumption existed that a claim that did not include the phrase “means for” did not trigger construction under 35 U.S.C. § 112(f) and this strong presumption was “not easily overcome.”4 The presumption could be overcome if the claim recited a non-structural, functional term without reciting sufficient structure for performing that function.5 Such terms are known as “nonce” terms and are simply substitutions for the term “means.”6

While preserving a baseline presumption that a claim without “means for” does not trigger construction under 35 U.S.C. § 112(f) as a means-plus-function limitation, the en banc portion of Williamson explicitly overruled that this presumption was “strong.”7 Williamson held that the “standard is whether the words of the claim are understood by persons of ordinary skill in the art to have a sufficiently definite meaning as the name for structure.”8 A claim that does not “recite sufficiently definite structure” for performing a function or else recites “function without reciting sufficient structure for performing that function” should be treated as a means-plus-function limitation.9

Given Williamson’s central holding that the presumption is no longer a strong one, courts will be more likely to find that claims not reciting “means” do not trigger means-plus-function.

Words that May Trigger Means-Plus-Function Construction

In a non-en banc portion of the Williamson decision, the Federal Circuit listed examples of nonce terms meriting means-plus-function construction. These nonces include “module” (the term actually at issue in Williamson), “mechanism”, “element”, and “device” along with “other nonce words that reflect nothing more than verbal constructs.”10

To date, the PTAB has applied this aspect of Williamson in at least one IPR proceeding. In Facebook, Inc. v. TLI Communications, Inc., the PTAB construed “digital pick up unit” as a means-plus-function limitation, despite the absence of the phrase “means for.”11 The PTAB reasoned that “unit” was so broad as to not convey sufficiently definite structure.12 The corresponding structure found in the specification was a digital camera.13 To the extent that these and similar terms have been used in issued patents, more claims may be construed as having means-plus-function limitations.

How to Determine Whether Means-Plus-Function Construction Applies

Even though an alleged nonce term is used in a claim, the PTAB, like district courts, will also consider whether the text of the limitation, including the nonce term, the prefix to the term, or the remainder of the claim, denotes structure. The PTAB is likely to consider extrinsic evidence in evaluating this text.

Consider the Full Text of the Claims

A determination that a given term is a nonce word does not end the inquiry. For example, the “prefix” that appears before a nonce word may provide the necessary structural meaning.1415 The PTAB has considered the role of the prefix of an alleged nonce. In Facebook, the PTAB decided that “digital pick up unit” was a means-plus-function element.16 The PTAB reasoned that “digital pick-up” was “not structural but functional”, and thus provided no structure sufficient to save the claim from means-plus-function handling.17

The analysis should also evaluate whether the text following the purported nonce includes details that would inform one of skill in the art of a sufficiently definite structure. In particular, petitioners and patent owners should consider how the purported nonce interacts with other elements, such as through inputs and outputs.18 In Williamson, the distributed learning control module was only claimed with high-level interactions, insufficient to denote any structure.19 In contrast, in Apple Inc. v. Motorola, Inc., a pre-Williamson case, the Federal Circuit held that the term “heuristic” was not a means-plus-function term because the claim also recited various inputs, outputs, and operations for the claimed heuristics.20 These included “the initial angle of a finger contact, the number of fingers making contact, the direction of movement of a finger contact, a specific swiping gesture, taping a certain location on the screen, or the angle of movement of a finger on the screen.”21

Consider Extrinsic Evidence

In determining whether one of skill in the art would consider an alleged nonce term to denote sufficiently definite structure, district courts often consult extrinsic evidence. Indeed, Williamson and prior cases openly invite expert declaration testimony on this issue.22 Some pre-Williamson decisions even consult dictionaries or other publications such as prior art patents to determine the structural import of an alleged nonce word.23 The PTAB is likely to follow the courts in relying upon extrinsic evidence for this determination.

Identify Corresponding Structure in PTAB Proceedings

When arguing that a claim should be interpreted as a means-plus-function limitation, a party should make such a designation in a claim construction portion of a PTAB petition or response. Significantly, the party should identify with particularity the structure, material, or acts of the specification that carry out the claimed function of the means-plus-function term.24 Both petitioners and patent owners might consider initiating arguments that a given term is a nonce word and subject to means-plus-function interpretation.

The PTAB recently dismissed an IPR petition for failing to identify the structure of “means” limitations in claim construction sections. In Apple Inc. v. ContentGuard Holdings Inc., the petitioner did not identify structure corresponding to recited “means.”25 The petitioner instead argued that there was no corresponding structure in the patent specification, and that the claimed “means” corresponded to any technique or scheme to perform the stated function.26 The PTAB found that the petitioner’s failure to identify structure for the “means for” term was fatal and denied institution of the IPR for those claims.27 As more claims will be construed as means-plus-function limitations after Williamson—sometimes unexpectedly—it is increasingly important for parties to note this requirement.

Indefiniteness Issues in PTAB Proceedings

An increased number of means-plus-function limitations may provide more opportunities for patent challengers to quickly end litigation through summary judgment of indefiniteness. In some cases, a patentee may have used a term—now found to be a nonce term—without sufficiently describing particular structures in the specification for performing the identified function. If sufficient structure that performs the function cannot be pinpointed in the specification, the claim is indefinite.28

Litigants have already combined Williamson and indefiniteness. For example, the Western District of Texas recently applied Williamson to construe terms “a system for detecting a failure in at least one of . . .,” “control device,” “processing device,” “communication device,” “video recording device,” and “monitoring device” as means-plus-function limitations.29 The court found that because the specification discussed no structure for performing the cited function of the claimed system, those terms were indefinite.30

A similar approach could apply in a CBM proceeding. For example, the petitioner in Epicor Software Corp. v. Protegrity Corp. argued that the claimed “means for comparing” was also indefinite because it was unsupported by sufficient structure in the specification.31 The PTAB agreed that these claims were more likely than not indefinite, as the only structure for performing the claimed function was a general purpose computer.32

However, indefiniteness is not a proper ground of invalidity in IPR proceedings. Furthermore, as discussed above, the petitioner in the IPR proceeding Apple Inc. v. ContentGuard Holdings Inc. argued that no structure corresponded to the recited “means for” terms, and the PTAB denied institution.33 In essence, the petitioner argued that the claims were indefinite. While petitioners might omit corresponding structure of a nonce term in order to preserve indefiniteness arguments in related litigation, this approach may be risky. Petitioners should evaluate whether any claims to be challenged include nonce terms, and whether identification of structure of these nonce terms would conflict with indefiniteness arguments in related litigation. If such a conflict exists, petitioners should consider choosing between pursuing an IPR or indefiniteness in related litigation.


Williamson may increase complexity in PTAB trials by causing more claims to be construed as including “means-plus-function” terms. Determining whether “means-plus-function” construction will apply to a given term is a multi-faceted problem governed by several factual issues that may require expert testimony. Furthermore, any growth in “means-plus-function” terms will consequently compound the issues to be briefed in petitions and responses, as well as foster the possibility of indefiniteness challenges.