Fig. 5.2 (a) One layer (layer A) of close-packed spheres contains hollows that exhibit a regular pattern. (b) A second layer (layer
B) of close-packed spheres can be formed by occupying every other hollow in layer A. In layer B, there are two types of hollow;
one lies over a sphere in layer A, and three lie over hollows in layer A. By stacking spheres over these different types of hollow,
two different third layers of spheres can be produced. The blue spheres in diagram (c) form a new layer C; this gives an ABC
sequence of layers. Diagram (d) shows that the second possible third layer replicates layer A; this gives an ABA sequence.