 Some supervenience relations are metaphysically (or logically) necessary. The property being a haircut or a halibut supervenes with metaphysical necessity on the two base properties being a haircut and being a halibut: two things cannot differ with respect to being a haircut or a halibut without differing either with respect to being a haircut or else with respect to being a halibut. Also, the surface areas of perfect spheres supervene with metaphysical necessity on their volumes (and vice versa) (Lombard 1986). Some supervenience relations are metaphysically contingent. Consider the Wiedemann-Franz Law, which entails that the electrical conductivity of a metal co-varies with its thermal conductivity. This law thus entails that electrical conductivity and thermal conductivity mutually supervene on each other. But on the assumption that the law is metaphysically contingent, the supervenience relation is too. It is only nomologically necessary that there cannot be a difference in one sort of conductivity without a difference in the other sort.