A type of antitrust analysis used to determine the legality of agreements (written or oral) between competitors. Under the per se rule, certain categories of agreements are presumed to violate antitrust laws, regardless of other factors such as business purpose or competitive benefits. The only relevant question under the per se rule is whether an agreement falls into one of these categories. Examples of categories held to be per se illegal are agreements to fix prices or output and agreements to divide territories or customers.

See also rule of reason.