For a given cation, Pauling defined the electrostatic bond strength to each coordinated anion as , where z is the cation charge and ν is the cation coordination number. A stable ionic structure is arranged to preserve local electroneutrality, so that the sum of the strengths of the electrostatic bonds to an anion equals the charge on that anion.
where  is the anion charge and the summation is over the adjacent cations. For simple solids, the  are equal for all cations coordinated to a given anion, so that the anion coordination number is the anion charge divided by each electrostatic bond strength. Some examples are given in the table.
Pauling showed that this rule is useful in limiting the possible structures to consider for more complex crystals such as the aluminosilicate mineral orthoclase, , with three different cations. However, from data analysis of oxides from the Inorganic Crystal Structure Database (ICSD), the result showed that only 20% of all oxygen atoms matched with the prediction from second rule (using a cutoff of 0.01).