Let us place a number of equal-sized spheres in a rectangular
box, with the restriction that there must be a regular arrangement
of spheres. Figure 5.1 shows the most efficient way in
which to cover the floor of the box. Such an arrangement
is close-packed, and spheres that are not on the edges of
the assembly are in contact with six other spheres within
the layer. A motif of hexagons is produced within the assembly.
Figure 5.2a shows part of the same close-packed
arrangement of spheres; hollows lie between the spheres
and we can build a second layer of spheres upon the first
by placing spheres in these hollows. However, if we arrange
the spheres in the second layer so that close-packing is again
achieved, it is possible to occupy only every other hollow.
This is shown on going from Figure 5.2a to 5.2b.