532 U.S. 23 (2001), United States Supreme Court

Facts
The plaintiff, Marketing Display, Inc., held a patent on a two-spring design to keep traffic signs standing in strong winds. After the plaintiff's patent expired, the defendant, TrafFix Devices, Inc., began manufacturing their own signs using the design. The plaintiff sued for trade dress infringement based on copying of the recognizable design.

Issue
The issue of the case concerned the legal question of whether trade dress protection could apply to the subject of an expired patent.

Result
The Court, in a unanimous opinion by Justice Anthony Kennedy, held that there can be no trademark protection for something that is functional because that would work as a detriment to competitors based on something other than reputation, which is the key consideration in trademark law.
The Court noted that the plaintiff has the burden of proving that the characteristic for which protection is sought is not functional—but having a patent for a design raises a very strong presumption that the design was functional. A design is functional if it serves any purpose that makes the product work better, or makes the product less expensive to produce. That an alternative design is available does not undercut the functionality of a given design.
Here, the design was clearly functional, and the plaintiff could not carry the burden of proving otherwise because the very characteristic that is sought to be protected by trademark is the one whose functionality was previously sought to be protected by patent.