Occasionally, a patentee will seek to define its invention with claims that recite a negative claim limitation – a specialized category of claim element that recites an element that is expressly and deliberately excluded.
By way of example, a claim directed to a stool with the limitation that the stool is “devoid of a backrest member” presents a negative claim limitation that is easy to understand. One seeking to challenge the patentability of such a claim would be expected to search for prior art that disclosed a backless stool. However, this raises a crucial question: What exactly must the challenger prove to show that the negative claim limitation is disclosed by the prior art? After all, the challenger bears the burden of proof with respect to each element of every challenged claim, and the patentee bears no burden of proving the opposite. Conceptually, proving that the prior art discloses a positively recited limitation is easy to do; but how does a challenger prove the existence of something that is, by its very nature, expressly nonexistent? This year, the Federal Circuit has considered this issue in two nonprecedential decisions, which provide some valuable guidance.
Earlier this year, in IBM v. Iancu, the Federal Circuit considered whether the Patent Trial and Appeal Board (PTAB) erred in finding that silence in the prior art was adequate proof of disclosure of a negative claim limitation. In relevant part, the claim at issue recited a method for managing user authentication by “triggering a single-sign-on operation.” During the inter partes review proceeding, the PTAB found that the use of the word “single” was a negative claim limitation that excluded two or more operations, and that a prior art reference that disclosed “associating a meCard with an accessCard” met the claim’s negative claim limitation because the reference was “silent as to what information is included in the access card,” such that it was not clear whether a single or multiple “sign-on operations” occurred.
On appeal, the Federal Circuit found that the PTAB had erred insofar as it found the negative claim limitation disclosed in the prior art despite finding that it could not tell one way or another whether the accessCard contained additional credentials corresponding to a second sign-on operation. The Federal Circuit emphasized that “[s]ilence in that sense would not by itself suffice for the Petitioner to meet its burden to prove, by a preponderance of the evidence, that there was no user authentication action in this scenario.” The decision in IBM appears to suggest that a challenger bears the burden of proving something more than mere silence to prove that a negative claim limitation is disclosed by the prior art.
Roughly five months after the IBM case, in WAG Acquisition v. WebPower, the Federal Circuit considered whether the PTAB had erred in holding that the prior art taught a negative claim limitation upon finding that the prior art reference taught that the limitation was optional or unnecessary. In relevant part, the claim at issue recited a server for distributing streaming media “wherein said server does not maintain a pointer into a buffer established within said server.” During the inter partes review proceeding, the PTAB found the claims anticipated over the patentee’s arguments that the prior art reference inherently disclosed the use of a pointer because the reference failed to disclose sufficient client-side control, thereby rendering the use of a pointer necessary.
On appeal, the Federal Circuit upheld the PTAB’s findings as being supported by substantial evidence, specifically finding that while the prior art reference did not show or describe a pointer, it emphasized client control, the lack of specialized server software and the use of pointer-less protocols, such that a pointer would be unnecessary. The WAG Acquisition case provides a clear example of a case in which the challenger was able to prove that the prior art described specific instances in which (and specific reasons for why) the missing pointer was impliedly optional or unnecessary. In this way, WAG Acquisition appears to stand for the proposition that the prior art need not expressly exclude an element in order for the challenger to satisfy its burden, but, rather, that exclusion of the element can be proven by affirmative evidence showing that the prior art suggests or describes instances in which the element is optional or unnecessary.
While both of the above-described Federal Circuit decisions were deemed nonprecedential, there appear to be at least a few takeaways for the prudent patentee or challenger to consider when a negative claim limitation is sought to be proven to be disclosed by the prior art:
- That a negative claim limitation is disclosed by the prior art is neither self-proving nor proven by mere silence in the prior art.
- A negative claim limitation may be proven to be expressly, impliedly or inherently disclosed by the prior art.
- A negative claim limitation may be proven to be disclosed by the prior art upon a showing that the prior art describes or suggests that the limitation is optional or unnecessary.
- Whether a negative claim limitation is optional or unnecessary must be proven by reliance on what the prior art says, including any teaching or suggestion that the limitation could be excluded therefrom.
- Though not its burden, the patentee can defeat an attempt to prove that a negative claim limitation is disclosed by the prior art by showing that the limitation is inherent or otherwise necessary.
 The USPTO expressly permits the use of negative claim limitations. See MPEP § 2173.05(i) (“So long as the boundaries of the patent protection sought are set forth definitely, albeit negatively, the claim complies with the requirements of 35 U.S.C. 112(b) or pre-AIA 35 U.S.C. 112, second paragraph.”); see also id. (“Any claim containing a negative claim limitation which does not have basis in the original disclosure should be rejected under 35 U.S.C. 112(a) or pre-AIA 35 U.S.C. 112, first paragraph, as failing to comply with the written description requirement.”).
 In the infringement context, the accused infringer bears the burden of proving invalidity by clear and convincing evidence. See 35 U.S.C. § 282 (“The burden of establishing invalidity of a patent or any claim thereof shall rest on the party asserting such invalidity.”); Microsoft Corp. v. I4I Ltd. P’ship, 564 U.S. 91, 95 (2011) (“[Section] 282 requires an invalidity defense to be proved by clear and convincing evidence.”). On the other hand, in a post-grant proceeding (e.g., an inter partes review proceeding) before the USPTO, the petitioner has the burden of proving a unpatentability under the lower preponderance of the evidence standard. See 35 U.S.C. § 316(e).
 See Fed. Civ. R. 32.1(b) (“An opinion or order which is designated as nonprecedential is one determined by the panel issuing it as not adding significantly to the body of law.”); see also Fed. Civ. R. 32.1(d) (“The court may refer to a nonprecedential disposition in an opinion or order and may look to a nonprecedential disposition for guidance or persuasive reasoning, but will not give one of its own nonprecedential dispositions the effect of binding precedent.”).
 Int’l Bus. Machs. Corp. v. Iancu, No. 2018-1065 (Fed. Cir. Apr. 1, 2019).
 See id. at *14-15.
 Id. at *15 (emphasis original).
 WAG Acquisition, LLC v. WebPower, Inc., No. 2018-1617 (Fed. Cir. Aug. 26, 2019).
 Where a claim element is not expressly disclosed by the prior art, it may nevertheless be shown that the element is inherent where the element is necessarily present, even if not shown or referred to in the reference. See, e.g., Endo Pharms. Solutions v. Custopharm, Inc., 894 F.3d 1374, 1381 (Fed. Cir. 2018).
 See WAG Acquisition, No. 2018-1617 at *12.
 See id. (“While [the prior art] does not specify that a pointer is not used, nothing in the record suggests that a pointer must be used”) (emphasis original).