Anions are larger than cations. Large sized anions occupy lattice sites, while small sized cations are found in voids. The ratio of radius of cation to anion is called radius ratio. In condensed matter physics and inorganic chemistry the cation-anion radius ratio (also: radius ratio rule) is the ratio of the ionic radius of the cation to the ionic radius of the anion in a cation-anion compound. This is simply given by . The table below gives the relation between radius ratio and coordination number, which may be obtained from a simple geometrical proof.