(i) When a third layer is placed over the second layer in such a way that the spheres cover the tetrahedral or ‘C’ voids, a three dimensional closest packing is obtained where the spheres in every third or alternate layers are vertically aligned (i.e. the third layer is directly above the first, the fourth above the second layer or so on) calling the first layer as layer A and the second layer as layer B, the arrangement is ABAB pattern or hexagonal close packing. When the third layer is placed over the second layer in such a way that spheres cover the octahedral or ‘d’ voids, a layer different from layers A and B is produced. Let us call it as layer C, continuing further, a packing is obtained were the spheres in every fourth layer will be vertically aligned. This pattern of stocking spheres is called ABCABC… pattern or close cube packing (CCP). 

(ii) A regular three dimensional arrangement of points in space is called crystal lattice. Unit cell is the smallest portion of a crystal lattice which, when repeated in different directions, generates the entire lattice. 

(iii) Wherever a sphere of the second layer is above the void of the first layer (or vice versa) a tetrahedral void is formed. These voids are called tetrahedral voids because a tetrahedron is formed when the center of these four spheres are joined. The voids which are surrounded by six spheres is called octahedral voids.