Now consider the hollows that are visible in layer B in
Figure 5.2b. There are two distinct types of hollows. Of the
four hollows between the grey spheres in layer B, one lies
over a red sphere in layer A, and three lie over hollows in
layer A. The consequence of this is that when a third layer
of spheres is constructed, two different close-packed arrangements
are possible as shown in Figures 5.2c and 5.2d. The
arrangements shown can, of course, be extended sideways,
and the sequences of layers can be repeated such that the