In United States trademark law, the functionality doctrine prevents manufacturers from protecting specific features of a product by means of trademark law. There are two branches of the functionality doctrine: utilitarian functionality and aesthetic functionality. Utilitarian functionality provides grounds to deny trademark protection to features which do something useful. Patent law, not trademark, protects useful processes, machines, and material inventions. Patented designs are presumed to be functional until proven otherwise. Aesthetic functionality provides grounds to deny trademark protection to design features which are included to make the product more aesthetically appealing and commercially desirable. Aesthetic features are within the purview of copyright law, which provides protection to creative and original works of authorship.
Functionality is met if :
A feature is essential to the use or purpose of the product; or
A feature affects the cost or quality of the product; or
Exclusive use of the feature would put competitors at a significant non-reputation related disadvantage
Essentially, functionality is grounds for denying trademark protection to a feature which grants a competitive advantage which is not related entirely to its function as a brand identifier. Trademarks serve to protect a firm's reputation and goodwill. The rationale behind functionality doctrine is that product markets would not be truly competitive if newcomers could not make a product with a feature that consumers demand.