Rock salt structure

The rock salt structure of ordinary sodium chloride has a coordination number of six for each kind of ion: six nearest neighbours of the opposite sign surround each ion along three perpendicular directions. Its conventional cubic unit cell contains four formula units; sharing corner, face, and edge sites between adjacent cells must be included in that count. Sixfold local coordination is compatible with $1:1$ overall stoichiometry.^[ref]

The checkerboard plane in the diagram is a slice through this three-dimensional arrangement. It leaves out the two nearest neighbours above and below the plane. Repeating a complete even-by-even patch gives equal numbers of signs; the displayed odd-by-odd crop has a boundary imbalance and is not a complete neutral ionic crystal or a unit cell. Other salts need not have the rock salt structure or the same coordination number.