Every IP right has its own definition of infringement. In this paper, we suggest that this diversity of legal rules is largely traceable to differences in the audience in IP cases. Patent, trademark, copyright, and design patent each focus on a different person as the fulcrum for evaluating IP infringement. The fact that patent law focuses on an expert audience while trademark looks to a consumer audience explains many of the differences in how patent and trademark cases are decided. Expert audiences are likely to evaluate infringement based on the technical similarity between the plaintiff’s and defendant’s works. Consumers, by contrast, are likely to pay more attention to market substitution and less attention to how things work under the hood. Understanding the different audiences in IP infringement is critical to understanding how the IP regimes define infringement.

The focus on audience has normative as well as descriptive implications. Neither patent law, with its focus on experts and technical similarity, nor trademark law, with its market-based consumer focus, has it entirely correct. Rather, we suggest that as a general matter infringement of an IP right should require both technical similarity and market substitution. Assessing infringement through the expert’s eyes ensures that the law prevents closely related works in the field while allowing later contributions to the field that are sufficiently different. The consumer vantage point ensures that we protect IP owners only when they have been harmed in the marketplace.

IP owners who want to show infringement should have to show both that the defendant’s work is technically similar to their own from the expert’s vantage point and that the defendant’s use causes the plaintiff market harm. Copyright law, which does look both to experts and to consumers at various points in infringement analysis, is on the right track.