r+/r- determines coordination geometry and structure type. 0.155-0.225: triangular (CN 3). 0.225-0.414: tetrahedral (CN 4, ZnS zinc blende). 0.414-0.732: octahedral (CN 6, NaCl rock salt). 0.732-1.0: cubic (CN 8, CsCl). NaCl: Cl- FCC lattice, Na+ in all octahedral voids, Z = 4, CN 6:6. CsCl: Cl- at corners (SC lattice), Cs+ at body centre, Z = 1, CN 8:8. ZnS: $S^{2-}$ FCC, $Zn^{2+}$ in half of tetrahedral voids (alternate ones), Z = 4, CN 4:4. CaF2 (fluorite): $Ca^{2+}$ FCC, F- in all tetrahedral voids, Z = 4, CN 8:4.