Proving Patent Infringement: Literal Reading, Equivalents, and the Limits of the Doctrine
By Casey Scott McKay ·
Infringement is the simplest question in patent law to state and the hardest to prove. Every limitation of a claim must be found in the accused product, and a single missing element ends the analysis. This article explains what the statute actually prohibits, why the all-elements rule structures every claim chart, how the doctrine of equivalents extends a claim and how prosecution history estoppel takes that extension away. It covers indirect liability and the knowledge requirement that makes a demand letter consequential, divided infringement across parties, the territorial limits that let manufacture abroad escape a domestic patent, and the marking rules that decide when damages start. It closes with the practical framing: infringement is an element-by-element proof exercise, and cases are lost on the element nobody charted carefully.
IP and Technology > Patent Litigation | Article | Published 15 June 2024 - Updated 2 June 2025 | Casey Scott McKay - marksy.us
Summary. Infringement is the simplest question in patent law to state and the hardest to prove. Every limitation of a claim must be found in the accused product, and a single missing element ends the analysis. This article explains what the statute actually prohibits, why the all-elements rule structures every claim chart, how the doctrine of equivalents extends a claim and how prosecution history estoppel takes that extension away. It covers indirect liability and the knowledge requirement that makes a demand letter consequential, divided infringement across parties, the territorial limits that let manufacture abroad escape a domestic patent, and the marking rules that decide when damages start. It closes with the practical framing: infringement is an element-by-element proof exercise, and cases are lost on the element nobody charted carefully.
Keywords: patent infringement, literal infringement, doctrine of equivalents, all elements rule, prosecution history estoppel, function way result, insubstantial differences, induced infringement, contributory infringement, divided infringement, joint infringement, claim chart, willfulness, marking, extraterritorial infringement, importation, method claims, system claims, direct infringement, accused product
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