Examples of behaviors that are typically considered to be per se violations of antitrust law:: Price Fixing; Collusion to Restrain Output; Division of Market Territories
Examples of behaviors typically judged according to the rule of reason.: Monopolizing; Mergers; Tying, Requirement, and Exclusive Dealing Contracts

Price fixing and collusion to restrain output are, essentially, the same behavior. As we know from simple demand and supply analysis, restricting output causes price to increase.