Filtration as a Framework, Not a Checklist: A Practical Guide from Practical Experience for Extrinsic Test Victory
Recently, firm client Moonbug Entertainment fully prevailed on appeal before the Ninth Circuit in its long-running copyright infringement claim against a cartoon copycat competitor. [1] One key issue the Court addressed was the filtration of unprotectable expression as part of the extrinsic test, the first part of the two-part test for infringement in the Ninth Circuit. Filtration of expression in application of the extrinsic test can often make or break a case, and it proved central to Moonbug’s victory both at trial and on appeal.
At its core, copyright law protects original, creative expression. But when approaching an infringement analysis, what the law excludes from protection is just as important as what it includes. When assessing a copyright claim, it is critical that potential plaintiffs understand these limitations on protection and weigh and test their claims against them. In this practical guide, based on our first-hand experience applying this test as counsel for Moonbug, we elaborate on how to get it right from the outset and through the finish line.
What is Filtration?
In the Ninth Circuit, a plaintiff asserting a copyright infringement claim must prove substantial similarity of the works under a two-test framework: the extrinsic test and the intrinsic test. The extrinsic test focuses on an objective comparison to assess whether there is substantial similarity of protected elements of a copyrighted work, including combinations of unprotectable elements where the selection and arrangement of those elements is sufficiently numerous and original to warrant protection. The intrinsic test then focuses on whether an ordinary, reasonable audience would find the works substantially similar in the “total concept and feel of the works.”
Because the extrinsic test evaluates similarity to protected elements of the copyrighted work, it requires the factfinder (e.g., the court at summary judgment or the jury at trial) to distinguish between protected and unprotected material, “filtering” out the unprotectable. This filtration is thus a legal framework by which the factfinder identifies and removes the unprotected elements before comparing the works at issue.
How do Ninth Circuit Courts Approach Filtration?
Because copyright protectability is a mixed question of fact and law, in cases that go to trial, courts provide jury instructions to guide the jury on how to filter out unprotected materials during the extrinsic test. Critically, courts do not and need not identify a comprehensive checklist or laundry list of unprotected elements that should or should not be excluded, and doing so often only invites problems. Harper House is a Ninth Circuit decision that establishes how courts approach this analysis, requiring only that jury instructions are adequate to ensure that the jury fully understands the issues. [2] In Harper House, a copyright infringement case that centered on a functional organizer with blank forms and tabs, the trial court instructed the jury that it “need not be concerned with the organizers’ copyrightability.” [3] In reversing the lower court’s decision, the Ninth Circuit noted that the plaintiff’s closing argument “fully exploited” this vague instruction and asked the jury to compare unprotectable, functional elements of the organizers. The trial court’s jury instructions as to copyrightability were therefore inadequate to ensure that the jury there fully understood the issues of protectable versus unprotectable elements. The Ninth Circuit therefore ordered a new trial for both infringement liability and damages.
Later cases applying Harper House confirm that courts do not need to identify a comprehensive laundry list of protectable or unprotectable elements in jury instructions. For example, in Dream Games, a case similar to Harper House focusing on a primarily functional work requiring virtual identity, not just substantial similarity, the court held that a court need not identify protectable elements.[4] The court in Dream Games reasoned that listing protectable elements would have “improperly limited the jury’s consideration” to those elements rather than the work as a whole. [5] And while the Dream Games court held that listing unprotectable elements “fairly and sufficiently explained the theory of the case and did not misstate the law,” the court did not hold that a list of unprotectable elements is a requirement. In Williams, the Ninth Circuit rejected an argument that a trial court “erroneously instructed the jury to consider unprotectable elements” in a musical composition by failing to list specific unprotectable elements. [6] And in Skidmore, the court held that a district court adequately instructed the jury to compare the works by determining whether “any ... musical elements that are original to Taurus ... also appear in Stairway to Heaven" without listing which elements were unprotectable. [7] These cases confirm that Ninth Circuit law does not require trial courts to list specific unprotected elements in filtering instructions.
If anything, Ninth Circuit case law teaches that presenting a jury with an enumerated list of specific protectable and unprotectable elements risks error by being over- or under-inclusive, thus confusing the jury more than guiding it. For example, in Mattel, in which the district court had instructed the jury using a list of specific protectable and unprotectable elements, the Ninth Circuit found error because the list of unprotectable elements was incomplete and not all of the listed protectable elements were, in fact, protectable. [8] The error required reversal because the jury could have found similarity based on unprotectable ideas not on the court’s list. [9]
Finally, recently, albeit in a non-precedential memorandum disposition, the Ninth Circuit confirmed that it “has never required district courts to identify for the jury specific, unprotected elements in the allegedly infringed work.” [10]
The key takeaway here is that extrinsic test filtration is a framework on which trial courts provide juries guidance, not an exhaustive checklist to be provided for jurors to tick through. Courts define a framework of what material is unprotectable, and juries then apply that framework to the specific facts of the given case, as set forth in the Ninth Circuit’s model instructions. [11] Were courts or copyright plaintiffs required to provide an exhaustive list, it would invade the jury’s role as factfinder and invite error.
Applying the Filtration Framework from the Outset to Drive Victory
With this foundation in place, approaching copyright litigation with extrinsic test filtration in mind from the start can be key to a copyright plaintiff’s victory, and those who fail to engage with it do so to their potential peril. Below are considerations to keep in mind as a copyright plaintiff (and counsel) over the life of a litigation where you expect application of the extrinsic test and, by extension, filtration of unprotectable elements, including by a jury at trial.
Evaluate the Strength of Your Claim: Before you ever file a complaint, you first need to assess the strength of your claim. If you believe a protected work was copied, you then need to determine the extent to which copied elements are protectable. Do you expect that application of the extrinsic test will require filtering out many elements of your work? If your work does include unprotectable elements, are they selected and arranged in a way that the combination is original protectable expression? Engaging with these considerations early on will force you to confront the strength of your claim and set you up for success from the start.
Complaint/Pleadings: Approach your complaint and other pleadings with the end game in mind: you will present to the court and jury. Determine early on what categories of protectable expression—e.g., character traits and development, distinctive visual or narrative structures, game level designs, dialogue, song lyrics, and sufficiently original and expressive selections and arrangements of otherwise unprotectable elements—apply to the work and draft the complaint with this in mind. Don’t shy away from explicitly acknowledging the unprotectable elements in the work, like common tropes, stock characters, game rules and mechanics, or individual dance steps, as appropriate to define your claim with credibility. But don’t concede any lack of originality or infringement of combinations of those elements that may be protectable as selected and arranged. Nor should you fail to highlight individual elements that are alone protectable and copied in the accused work. This early framing of your claim, done correctly, can inform and shape everything that follows, including what elements or combinations thereof survive filtration during the extrinsic test.
Fact Discovery: Shape fact discovery with the goal of seeking admissions that support your claim as you have shaped it. For example, seek admissions regarding elements of your work that are protectable. If there are elements of your work that are unprotectable, use fact discovery to bolster any allegations you have that those elements are protectable as a sufficiently original selection and arrangement. Such evidence can prove invaluable when you present it to the fact finder later in the case, whether at summary judgment or trial.
Expert Discovery: Consider engaging an expert witness that has the background and experience to help develop a solid filtration analysis. This is a major tool in your arsenal because expert witness testimony is vital to helping the fact finder perform filtration and ultimately find infringement. A good expert should identify unprotectable and protectable elements, explain why the remaining filtered elements are original and protectable (or original as a combination), and compare only that filtered expression. Additionally, the expert should avoid “kitchen sink” or garbled comparisons that fail to filter and are likely to invite criticism or exclusion.
Summary Judgment: At summary judgment, present the court with ready-made filtered comparisons through expert testimony. This will force the defendants to argue why the filtered similarities are insufficient for a finding of infringement on summary judgment. You should also be prepared for the defendant to attack the expert’s analysis and filtration and have a response strategy ready. Recall that to succeed on a summary judgment motion, the party must demonstrate there is no genuine issue of material fact and that it is entitled to judgment as a matter of law. Winning on infringement at summary judgment is very rare in the copyright context in the Ninth Circuit under the intrinsic test. But, using filtration to make a strong showing on the extrinsic test can fortify your chances of securing summary judgment on infringement at least on the extrinsic part of the test, and if opposing a party seeking summary judgment of non-infringement, create sufficient fact questions to avoid a premature end to your case and ensure it goes to a jury.
Motions in Limine: If issues of filtration were determined on a motion for summary judgment but the case proceeds to trial, consider using pretrial motions to try to limit any revisitation of filtration arguments during presentation of evidence before the jury that would only confuse them.
Jury Instructions: This is the big one. Lock in the court’s trial framework for filtration under the extrinsic test (i.e., categories of unprotectable expression, combinations of unprotectable elements that can together garner protection) before trial with jury instructions and motion practice. This way, by the time the trial starts you can tailor your presentation of the evidence to what you know the court’s guidance will be. By taking this approach, you’ll ensure the jury understands how to identify what to filter out and what to keep in during application of the extrinsic test, including how to think about and address protectable combinations of elements. Then, after all of the evidence is in, revisit the instruction to conform to the evidence as it relates to protectable and unprotectable elements to give the jury sufficient guardrails to properly perform filtration.
Conclusion
Extrinsic test filtration is a doctrine that rewards a copyright plaintiff’s preparation, specificity, and discipline. Done correctly, filtration will clarify, rather than dilute, claims that allege copying of core expressive material so that the strong originality of your client’s work stands out. Approach filtration proactively and thoughtfully, not in reaction to what the other side might argue, and tailor your case accordingly. Doing so carries significant potential rewards, including a well-instructed and guided factfinder that can marshal the evidence you’ve presented and deliver you a verdict.
***
[1] Moonbug Entertainment Ltd. v. BabyBus Co., Ltd., No. 3:21-cv-06536-EMC (N.D. Cal. 2023), aff’d No. 24-3748 (9th Cir. Nov. 6, 2025). See our earlier write-up on this case here: https://www.tyzlaw.com/news-archive/tyz-law-group-secures-ninth-circuit-victory-for-firm-client-moonbug-entertainment-following-cocomelon-copyright-trial-win.
[2] Harper House, Inc. v. Thomas Nelson, Inc., 889 F.2d 197, 205 (9th Cir. 1989).
[3] Id. at 198–99.
[4] Dream Games of Ariz., Inc. v. PC Onsite, 561 F.3d 983, 989 (9th Cir. 2009).
[5] Id.
[6] Williams v. Gaye, 895 F.3d 1106, 1124–25 (9th Cir. 2018).
[7] Skidmore v. Led Zeppelin, 952 F.3d 1051, 1076–77 (9th Cir. 2020).
[8] Mattel, Inc. V. MGA Ent., Inc., 616 F.3d 904, 916 (9th Cir. 2010).
[9] Id.
[10] Moonbug Ent. Ltd. v. BabyBus Co., Ltd., No. 24-3748; 24-6757, 2025 WL 3111597, at *1 (9th Cir. Nov. 6, 2025).
[11] See Manual of Model Civil Jury Instructions, Model Instruction No. 17.19, https://www.ce9.uscourts.gov/jury-instructions/node/276.