The functionality doctrine prevents trademark law, which seeks to promote competition by protecting a firm's reputation, from instead inhibiting legitimate competition by allowing a producer to control a useful product feature. See Qualitex Co. v. Jacobsen Products Co., Inc., 514 U.S. 159 (1995). A product feature is considered functional, and cannot serve as a trademark, if it is essential to the use or purpose of the article or if it affects the cost or quality of the article. See Id. The doctrine prevents the use of a product feature as a trademark where such use would put competitors at a significant non-reputation-related disadvantage.