Patent Litigation: An Introduction to Patent Claims, Limitations, Infringement, and Invalidity

Six-part introduction to software patent litigation

This series on patent litigation, published at, focuses on factual issues, especially in software patent litigation. The series covers patent claims; the so-called “limitations” (elements of a device, or steps of a process) that comprise claims; infringement; and invalidity.

Part 1

  • A patent by itself does little, and needs enforcement primarily through litigation or credible threat of litigation.
  • The patent property is largely a right to sue others for infringement, assuming that the patent owner has a plausible (not necessarily correct) case that defendant is infringing.
  • Patent infringement is similar in some ways to trespassing on land.
  • The actionable part of a patent is its “claims.”
  • A patent claim is like a collection of stakes marking out the perimeters of a patented invention.

Part 2

  • Patent claims are made up of limitations, which are selected elements (components of a device) or steps (of a process) implementing an invention.
  • Method claims are comprised of steps in a process; apparatus/device claims are comprised of elements (components).
  • Analyzing a simple claim for printer toner
  • “Claim construction” uses the patent specification (and a hierarchy of other sources) to interpret the meaning of terminology in a claim.
  • Broad and narrow claim scope: the size of a patent claim is an indication of claim scope (i.e., whether a claim protects a large or small area of technology); adding limitations generally reduces claim scope, but generally increases likelihood of validity.
  • Sub-limitations: similar to how patent claims are made up of limitations, limitations are often further made up of sub-limitations, and/or of attributes or features.
  • The sub-parts or attributes/features of a limitation can be used as bases for comparisons performed to determine whether the limitation is or is not present elsewhere (e.g. in an accused product or in an asserted piece of prior art).
  • Patent claims serve as devices for testing patent infringement and invalidity.
  • The set of limitations comprising a patent claim is not a complete list of elements or steps, but is instead the subset necessary to differentiate the claim from prior art, while still trying to leave a wide scope of infringement.
  • The boundaries of patent property are determined in part by steering between infringement and invalidity.

Part 3

  • Defending against an accusation of patent infringement
  • Analyzing a software-patent claim
  • Negative limitations (looking for absences)
  • Interconnections between limitations
  • Using a patent claim as a parts list or checklist to investigate infringement by searching for products that include the so-called “limitations” (elements) of the claim.
  • Defending against a patent owner’s assertion of infringement by showing a product’s absence of one or more limitations of an asserted claim, and/or by showing the patent claim is invalid (e.g., by finding each and every limitation in a piece of prior art).
  • Nomenclature (terminology): the names the patent owner uses for claim limitations, and the often-different names used for matching elements found in accused products or in the prior art.
  • A patent claim often changes during examination by the patent office (PTO); why such changes — often made to avoid the prior art — are important for understanding the scope of the claim.
  • Why the drawing on the front page of a patent isn’t necessarily a good representation of a patent claim.
  • Why the title of a patent, or what otherwise seems like what the patent is “all about,” may not be the focus of patent litigation.

Part 4

  • Investigating possible infringement (finding candidates for closer inspection)
  • Limitation-by-limitation comparison of a software-patent claim to an accused product
  • “Functional claiming” (actually, claims with functional language; means-plus-function)
  • Function vs. structure in patent law
  • Equivalence (DoE) and function/way/result test
  • Claim-to-product comparisons are limitation-by-limitation
  • Patent infringement analysis generally requires disaggregated comparisons using the claims of a patent, the limitations of a claim, and the attributes of a limitation.
  • Patent litigation requires thinking about how language maps onto technology; software in particular has looser nomenclature than other fields such as chemistry or electronics.
  • Patent owners (P) suing for infringement must conduct a diligent search for detailed publicly-available information about how the defendant (D)’s accused product works, and about the location in D’s product where each of P’s claim limitations likely reside; this diligent search often includes reverse engineering to learn product internals.
  • “Reading on” means using a patent claim as a template to view an accused product or service, to determine if the product/service infringes the patent claim.
  • Using claim construction in infringement analysis: reading the construed claim (not the raw claim language) onto an accused product: while “importing” from the patent specification into the claims is a cardinal sin in patent law, so is trying to work with raw unconstrued claim language; claim construction may in effect expand the claim language.
  • Comparing claim limitations with an accused product or prior-art reference should not be mindless keyword searching or “pattern matching.”
  • In the search for patent infringement, a key question to ask is: If someone were infringing, what terminology would they be using for the different components or steps that comprise infringement, i.e., that correspond to the limitations of the asserted patent claim?
  • The choice of which parts of a product are juxtaposed with a specific claim limitation is constrained by how the limitation fits into the claim as a whole.
  • A typical argument in patent litigation might be whether port 80 (HTTP), when used solely for local, is nonetheless a network port.
  • Saying that X is-a Y or X is-not a Y, is the sum of many smaller comparisons, which can be spelled out: “all evidence of identity derives its power from the aggregation of individually uninteresting matches or non-matches.”
  • The assertion that D’s product attribute Y matches P’s claim limitation X can be structured, based on attributes of the claim limitation.
  • Using “means for” and other forms of functional claiming requires taking corresponding structural details from the non-claims portion of the patent specification, and finding at least one such structure in an accused product.
  • The Doctrine of Equivalence (DoE) is a fallback from literal infringement, but is crucial to patent law; similarly, Part 5 shows that obviousness is a fallback from anticipation (lack of novelty), yet plays a larger role in patent law than one might think from this fallback role.
  • There are well-established tests for equivalence, such as function/way/result (FWR), which are applied to each limitation (not to a patent claim as a whole); there are restrictions on what P can assert is an equivalent.
  • The patent-law “nose of wax”: P can’t twist its claim one way to get the patent granted (or to later evade invalidity), and then twist it a different way to capture infringement.
  • The invalidity/infringement near-mirror image helps inhibit litigants from taking unreasonable positions.
  • “After-arising” technology: how much should a pioneer be able to reach-through to capture later improvements?
  • Improvements, by making an original basic invention actually work on a large scale, may turn out to be as important as the basic invention.

Part 5

  • Investigating possible invalidity of a patent claim
  • Limitation-by-limitation comparison of a software-patent claim to an asserted piece of prior art
  • “File wrappers” (prosecution history) and claim amendments
  • Patent invalidity is always at issue (no “quiet title”)
  • The concept of patent invalidity: while patents granted by the government (in the U.S., the PTO) are presumed valid, and while this presumption of validity forms a large part of the value of a patent, nonetheless this presumption can be rebutted (with a sufficiently high level of proof), and claims of the patent found to be invalid.
  • Patent claims can be invalidated by showing:
    • anticipation (lack of novelty),
    • “obviousness” (a term of art with specific meaning in patent law),
    • lack of “enablement” (failure to sufficiently disclose how to make and use a claimed invention), and
    • on-sale and public-use bars; public disclosure outside one-year US “grace period” (patent owner can trip itself up with its own too-early disclosures or public uses).
  • Invalidating a patent claim by showing anticipation (lack of novelty) depends on the “prior art.”
  • Investigating prior art for a software patent claim: what counts as “prior art.”
  • How a patent claim is compared with prior art, to determine if the claim is invalid for lack of novelty
  • Checking prior art already found by the PTO (while it was examining a patent application), as reflected in the patent’s “file wrapper.”
  • Performing a limitation-by-limitation comparison of a specific prior-art reference with a patent claim.
  • Using obviousness as another way to invalidate a patent: looking for the ordinary non-inventive skilled practitioner (POSITA)’s “motivation to combine” multiple prior-art references.
  • How a patent owner can respond to assertions that its patent claim is invalid for anticipation or obviousness.
  • Patents as “shaky” property, due to the almost ever-present risk of patent invalidation.

Part 6

  • Claim charts for analyzing infringement and invalidity
  • The need for a reasonable pre-filing investigation (Rule 11) and “plausible” infringement contentions (Iqbal/Twombly standard) when bringing a patent infringement case.
  • Local Patent Rules in key federal districts, focusing on court requirements for infringement and invalidity contentions and responses.
  • Discovery and mandatory disclosures in litigation: getting information the other side has.
  • Very briefly touching on the many issues we’ve managed to skip over, despite the length of this introduction to patent litigation, including: venue and “forum shopping”; the Markman claim construction hearing; summary judgment and “failure of proof”; settlement or trial; remedies (injunctions and monetary damages); willful infringement and attorney’s fees; design-around; importation, “domestic industry,” and the ITC; and appealing adverse rulings to the Court of Appeals for the Federal Circuit (CAFC).



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