If we imagine metal atoms as being hard balls, when densely packed in two dimensions each ball will be in contact with six other balls (A). When another layer of this 2 dimensional arrangement is placed on top of the first, the packing will be densest and the structure most energetically stable when the metal atoms are placed on top of the hollows (B) of the first layer. When a 3rd layer is placed on top of the 2nd layer, there are two possibilities. Namely, the 3rd layer (A) overlaps with the 1st layer (A) or the 3rd layer (C) overlaps with neither (A) nor (B). The ABAB...-type packing is called hexagonally close-packed (hcp) (Fig. 2.2), and the ABCABC...-type is called cubic close-packed (ccp) (Fig. 2.3). In both cases, each ball is surrounded by 12 balls, that is, it is 12-coordinated. The coordination polyhedron of hcp is anti-cubooctahedron, and that of ccp is cubooctahedron. When the lattice is sliced in different planes, the unit lattice of ccp appears to be a face-centered cubic lattice (fcc), containing a ball at each cubical apex and on the center of each face (Fig. 2.4). The unit lattice of hcp is a rhombohedral prism in which two balls are located in the positions shown in (Fig. 2.5). There are several different modes of piling up layers other than the normal hcp and ccp,