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Conventional antitrust norms analyze market power—as a stepping stone to anticompetitive effects and, hence, prohibited conduct—from the perspective of product substitutability. Two goods or services are said to compete with one another when they are reasonably interchangeable from the perspective of consumers, or to put it in more formal economic terms, when there is cross-elasticity of demand between them. Conversely, when two goods or services are not reasonably interchangeable, they are not horizontally related and are said not to compete with one another. Since a concern over horizontal agreements and horizontal effects dominate antitrust—courts even analyze vertical agreement or merger cases in terms of their horizontal effects—proof of effects among substitutable goods or services is a crucial touchstone of antitrust analysis.