In ionic solids, the ratio of radius of cation to radius of anion is called radius ratio.

$$Radius~ratio=\frac{Radius~of~cation~(r^{+})}{Radius~of~anion~(R^{-})}$$

Since, cations have a tendency to get surrounded by the maximum number of the anion. Therefore, larger the cation radii, the greater will be its coordination number.

The relationship between radius ratio, the coordination number and structural arrangement is called radius ratio rule.