In a footnote to its 73-page decision, the Ninth Circuit — which heard the appeal “en banc,” or as a full panel of 11 judges — also explained what constitutes illegal copying when it comes to works that involve generic or commonplace elements. In those cases, the judges said, only a minimal, or “thin,” level of copyright applies, and a plaintiff must show that a work is “virtually identical” to a defendant’s.

The author of the panel’s majority opinion, Judge M. Margaret McKeown, gave no specifics about what kinds of works may apply, just that they must be virtually identical “if the range of protectable expression is narrow.” But lawyers, and at least one judge, seized on that statement as applying to brief musical passages that may recycle common chords or melodies — exactly the situation with Perry’s hit “Dark Horse,” which a jury last summer found had infringed on an eight-note instrumental pattern in a Christian rap song.

Just a week after the Led Zeppelin decision, the judge in Perry’s case, Christina A. Snyder of Federal District Court in Los Angeles, cited the Ninth Circuit’s footnote in a ruling that threw out the “Dark Horse” jury’s verdict — and, with it, a $2.8 million damages award. Those eight notes were “not a particularly unique or rare combination,” Judge Snyder wrote, and therefore could not be protected by copyright. (Lawyers for the plaintiff, Marcus Gray, who performs under the name Flame, have said they will appeal.)