It will be noticed that the atoms in the cube faces of the ccp structure lie in a square array like that in Fig 1.3(b) and the ccp structure may be constructed by stacking these layers such that alternate layers lie in the square interstices marked X in Fig. 1.8 (a). The models show how the four close-packed layers arise like the faces of a pyramid(Fig1.8 (b). If, on the other hand, the layers are all stacked directly on top of each other, a simple cubic structure is obtained(Fig 1.8(c)). This is an uncommon structure for the same reason as the simple hexagonal one is uncommon. An example of an element with a simple cubic structure is polonium.