This article concerns trademark law's functionality doctrine and the Supreme Court's troublesome opinion concerning it in TrafFix Devices, Inc. v. Marketing Displays, Inc. The doctrine provides that if a producer's useful or aesthetic design feature is "functional," then competitors can lawfully copy it even if the feature otherwise would be protected against copying by trademark principles. In order to introduce the functionality doctrine and the trouble with TrafFix, it is helpful to describe the nature of design features, the simultaneous roles they may play as source-identifying trade symbols and as useful or aesthetic product elements, and trademark law's place in the United States' legal system that includes patent law and reflects national policy favoring competitive markets.

Part One of this article briefly defines some terms employed throughout and provides a short review of trademark principles. Parts Two and Three employ different methodologies to illuminate the functionality doctrine and the significance of TrafFix. Part Two develops the etymology of important functionality standards including those considered in TrafFix. It highlights links among the standards, and between the standards and evidence of functionality. Part Two's etymology suggests Part Three's economic methodology. The strength of the economic lens amplifies and simplifies the doctrine while its limitations help to explain why the functionality doctrine has been problematic and why TrafFix is troublesome. After summarizing the lessons of Parts Two and Three, Part Four discusses how courts may decide functionality cases with care, reason, and sensitivity; and how TrafFix may affect this process. Part Five concludes the article with thoughts concerning TrafFix's future impact on trademark law and the functionality doctrine.