INTRODUCTION 1826 I. A BRIEF HISTORY OF TRADEMARK FUNCTIONALITY 1834 A. Pre-Lanham Act Cases 1834 B. From the Lanham Act to Qualitex 1840 C. TrafFix and the Present 1844 II. [...], According to functionality doctrine, trademark protection cannot be granted for any feature that is essential to a product's use or purpose, or that affects the product's cost or quality. But because of the placebo effect, even seemingly inert names and symbols are imbued with precisely this kind of power. In fact, a wide variety of real-world phenomena challenge the prevailing understanding of trademark functionality, from the social uses of high-fashion marks to the cost reductions enabled by certification marks. More fundamentally, a valuable trademark of any kind should act to reduce search costs for consumers and improve quality through reputation. And yet, rather than leading to invalidation, all of these well-documented functionalities are apparently tolerated by trademark law--sometimes merely ignored, but often celebrated explicitly. This Article proposes a more unified theory of functionality: fragility. Some product features affect cost, quality, use, and purpose in ways that are non-fragile--the effects would persist even if every producer were to copy the same feature. But some features affect the product in ways that are fragile--the effects would be degraded or broken through unchecked copying. In reality, only non-fragile functionalities are actually prohibited, whereas fragile functionalities are permitted and even encouraged. In a manner surprisingly similar to patent or copyright law, trademark law appears to carefully distinguish between improvements that require its protection in order to manifest, and those that do not. This fragility theory not only better explains real-world case outcomes and the functionality doctrine's full history, but also offers a conceptual improvement that can be applied to all types of trademarks. A generic term, for example, exhibits non-fragile linguistic functionality. Moreover, fragility theory provides consistent answers to divisive boundary issues in trademark law, such as overlapping protection under copyright, anti-dilution rights, and post-sale confusion doctrine. At the same time, recognizing this fragility pattern calls attention to potentially adverse consequences in terms of distributive justice and market competition--consequences that trademark law itself may not be able to remedy.