A quantum mechanical treatment of the conduction electrons of a metal in a polarisable interface shows that they can make an appreciable contribution to the electrical capacitance. Results for six metals are given, showing how differences in metal properties account qualitatively for experimentally observed differences in interfacial capacities, when the solvent is replaced by a dielectric film. To justify the neglect of solvent structure when metal properties are treated, the coupling between metal and solvent is discussed for orientable point solvent dipoles, and for a representation of the solvent polarisation by a pair of charged planes. The electron profile affects the polarisation of the solvent near the point of zero charge, but the solvent structure has an almost negligible effect on the metal contribution to the capacity. One parameter in our model, the distance from metal ions to the first solvent layer, can be expected to vary as the interface is charged, due to changed bonding. Coupling by such an effect can be quite important, and severely decreases the variation of metal capacity with charge.