Because like charges repel, it is necessary to prevent two anions or two cations from coming in contact with each other. Consequently there are many exceptions to the simple rules discussed here. In most ionic solids the cation and anion are different in size. The larger ion, which is typically the anion, packs with other ions to yield a particular structure, which is often a closest-packed structure. The smaller ion, typically the cation, then fits into the holes between the larger ion.
Holes come in a variety of geometries and sizes.
Each atom in a crystal is surrounded by a certain number of ions of opposite charge. This number is called the coordination number for the ion. Links are provided to pages illustrating several common hole geometries. In viewing the displays of the trigonal, tetrahedral, octahedral, and cubic holes, it should have been apparent that the holes different in size. The display below shows the relative sizes of the largest spheres that can fit into a hole of each geometry.