In a crystal containing different cations, those of high valency and small coordination number tend not to share polyhedron elements with one another. This rule tends to increase the distance between highly charged cations, so as to reduce the electrostatic repulsion between them.
One of Pauling's examples is olivine, , where M is a mixture of  at some sites and  at others. The structure contains distinct  tetrahedra which do not share any oxygens (at corners, edges or faces) with each other. The lower-valence  and  cations are surrounded by polyhedra which do share oxygens.