One of the features that determines the type of hole a cation will occupy is the radius ratio, which is determined from the ionic radii of the cation and anion. The radius ratio is equal to the ratio of the radius of the cation, r₊, to the radius of the anion, r_- .

Radius ratio = r₊ / r_-

Click the link on the right to show the radius ratio. The underlined text above links to a definition.