NaCl is a AB type of solid. Its crystalline structure is known as rock salt structure in which ${\mathrm{r}}_{\mathrm{Na}}^{+}$and ${\mathrm{r}}_{\mathrm{Cl}}^{-}$ are 95 and 181 pm respectively, where Cl^- is present at the lattice point of face centered cubic unit cell Na ions are located in void and those compounds which have same crystalline structure also have same coordination system in which Na⁺ and Cl^-ions are surrounded by certain number of opposite ions respectively. Under high pressure coordination number changes to 8: 8 type from 6 : 6 type.