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An isotropic, incompressible elastic material is characterized by just one parameter, its shear modulus. When two such materials are mixed on a fine scale, an elastic composite results, always incompressible but generally anisotropic. This effective or homogenized material is described by its stress-strain tensor Cokt, a symmetric linear map on the space of trace-free symmetric matrices, or equivalently by its effective energy quadratic form (C~, O. Our goal is to bound this quadratic form above and below in terms of the shear moduli#~