The Fisher-Freidman definition classifies commodities as substitutable of complementary depending on the global shape of indifference maps. It thus requires the specification of the global mathematical properties of any model used to illustrate this definition. We utilized the simple model of a direct utility function of quadratic homogeneous form. As shown in Sections II-V, the substitution effect--theAllen elasticity of substitution-is always non-negative with this specification.Therefore, by the Hicks-Allen-Shultz definition, this model applies only to competing commodities and not to complementary commodities, a conclusion that is independent of the number of commodities.