Other interesting arrangements can arise when polyhedra overlap such that not only are some vertices shared, but also the central atom of one polyhedron also serve as a vertex for the other polyhedron. This is called interpenetration. Here we see example, two icosahedra interpenetrated such that the one vertex of each lies at the center of their neighboring polyhedra. Around the outer surface of the unit, the two polyhedra share a pentagonal ring of vertices. To make two filled icosahedra, we would need 2 × (12 + 1) = 26 atoms. In this case, however, 7 atoms are shared between the polyhedra. If you count the atoms, you should find only 19.