Proving Copyright Infringement: Access, Substantial Similarity, and the Idea-Expression Divide
By Casey Scott McKay ·
A copyright plaintiff must prove two things, ownership of a valid copyright and copying of constituent elements that are original, and the second splits into two very different inquiries that litigants routinely blur. This article walks through the whole machine: what Feist requires of originality, how Section 102(b) and the merger and scenes a faire doctrines shrink the protected core of a work, how copying in fact is proven through access plus probative similarity or through striking similarity alone, and how unlawful appropriation is judged under the Ninth Circuit's extrinsic/intrinsic test, the Second Circuit's ordinary and more discerning observer tests, and the abstraction-filtration-comparison method built in Altai. It explains why the Ninth Circuit buried the inverse ratio rule in Skidmore, why deposit copies decided both the Stairway to Heaven and Thinking Out Loud trials, and how music, photography, fabric, architecture, and software each generate their own recurring fact patterns. It covers the procedural chokepoints where these cases are really won: Rule 12(b)(6) dismissals on the works themselves, summary judgment under the extrinsic test, Daubert fights over musicologists, and the jury instructions that decide whether filtration survives to the verdict. Finally it maps the timing rules of Section 507(b), Petrella, and Warner Chappell, including the question about the discovery rule that the Supreme Court has still not answered.
IP and Technology > Copyright | Article | Published 26 March 2025 - Updated 7 April 2026 | Casey Scott McKay - marksy.us
Summary. A copyright plaintiff must prove ownership of a valid copyright and copying of constituent elements that are original — and that second element splits into two very different inquiries litigants routinely blur. This article works through the whole machine: what Feist requires of originality, how 17 U.S.C. § 102(b) and the merger and scènes à faire doctrines shrink a work's protected core, how copying in fact is proven through access plus probative similarity or striking similarity alone, and how unlawful appropriation is judged under the Ninth Circuit's extrinsic/intrinsic test, the Second Circuit's ordinary and more discerning observer tests, and the abstraction-filtration-comparison method built in Computer Associates v. Altai. It explains why Skidmore buried the inverse ratio rule, why deposit copies decided both the Stairway to Heaven and Thinking Out Loud trials, where the recurring fact patterns go wrong, and where these cases are actually won procedurally. It closes on the timing rules — § 507(b), Petrella, and Warner Chappell Music v. Nealy — and the question the Supreme Court has still not answered.
Keywords: copyright infringement · substantial similarity · idea-expression dichotomy · access and probative similarity · striking similarity · extrinsic intrinsic test · ordinary observer test · more discerning observer · abstraction filtration comparison · merger doctrine · scènes à faire · feist originality · skidmore v led zeppelin · computer associates v altai · petrella laches · warner chappell v nealy · discovery rule copyright · statutory damages · copyright registration prerequisite · de minimis copying
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