Hexagonal close packing : The spheres of the `3^(rd)` layer are exactly aligned with those of first layer . This pattern of spheres is repeated in alternate layers . Tetrahedral voids of the `2^(nd)` layer may be convered by the spheres of `3^(rd)` layer . This structure is called hexagonal close packed (hcp) structure .
Cubic close packing : The spheres of `3^(rd)` layer cover the octehedral voids of `2^(nd)` layer . But the spheres of `4^(th)` layers are aligned with those of first layer . This structure is called cubic close packing .