Journal of Statistical Physics, Vol. 46, Nos. 1/2, 1987. p. 161-177.
A complete geometrical characterization is given of the set of all possible effective conductivity tensors for two-dimensional composites made of an arbitrary number of given anisotropic phases. It is rigorously established that any polycrystalline composite formed from an arbitrary (possibly infinite) number of phases can be replaced by a composite formed from only two of the given phases without altering the effective conductivity tensor.