Many structures are based upon the close-packing of atoms or ions into hexagonal layers. In a close-packed structure, each atom or ion is surrounded by six others, resulting in very efficient packing. These hexagonal layers, in turn, may be packed in two different ways, giving rise either to a hexagonal close-packed structure, or a cubic close packed structure.

In one type of structure the hexagonal layers are stacked in an ABC fashion, so that the fourth layers lies immediately over the first, the fifth layer lies immediately over the second, and so forth.. This is emphasized by the vertical lines, which connect the first and fourth layers. The ABC stacking of hexagonal layers gives rise to the face-centered cubic structure, and is discussed more thoroughly in a separate section

Hexagonal layers may also stack in an ABAB fashion, as shown in this illustration. Note that the two red layers lie immediately above each other, as do the two blue layers. The longer vertical lines connecting the layers are shown for clarity.Whereas the satking of layers in an ABC gives rise to a face-centered cubic structure, the stacking of layers in an AB fashion gives rise to a hexagonal structure.

Regardless of whether hexagonal layers are stacked in an AB or ABC fashion, there exist two types of spaces or holes between the layers. One type of space is called an octahedral hole, and is formed between three atoms in one layer and three atoms in the layer immediately above or underneath. Although it takes six spheres to form an octahedron, the name is derived from the fact that the resulting shape has eight sides. 

A second type of space which can exist between stacked hexagonal layers is called a tetrahedral hole. Tetrahedral holes are formed between three atoms in one layer and a single atom immediately above or underneath.