From the packing fraction data, it is clearly evident that there is more empty space in primitive cubic and body centered cubic unit cells than in the face centered cubic unit cell. This implies that atoms in FCC unit cell are more closely packed than in PC and BCC unit cell. Thus FCC unit cell has the closest packing. The hexagonal primitive (HP) unit cell, also has the closest packing as its packing fraction matches with that of FCC unit cell. Thus, FCC and HP unit cells has the most efficient arrangement of atoms called closest packing. Let us start with the structure shown in fig.1, which is called layer `A`. If we focus on the only enclosed atom, we can notice that each atom is surrounded by six atoms in this layer. There are certain spaces left in between the atoms. These spaces are called voids or grooves. Now, we place an atom (of the same type) into the groove (depression) between the atoms in the first layer, then the adjacent groove gets partially blocked by this atom and we are unable to put the atom in the adjacent groove. So, the atoms of the second layer can only be placed in the alternate grooves between the atoms in the first layer. This constitutes second layer called as layer `B` shown in fig.2. In layer `B` also, each atom is surrounded by six atoms. Thus, layer `A` and `B` are identical, with the only difference in their placement. The layer `B` has two types of voids, marked as `x` and `y` voids. Thus, there are two ways that a third layer atom may cover the second layer to achieve closest packing.

The atoms may fit into the voids `x` such that each third layer atom is directly over a first layer atom. Thus, if we put the atoms on all the `x` type of voids, they would eclipse all the atoms of the `A` layer as shown in fig.3. This means that there would be no difference between the arrangement of the first and third layer, so we can call the third layer as layer `A`. Thus, the layering pattern generated is `ABABAB`, which is known as hexagonal close packed (HCP) structure, which corresponds to hexagonal primitive (HP) unit cell.

Alternatively, the third layer atoms may fit into the `y` type of voids that lie directly over the voids in the first layer as shown in fig.4. The third layer formed in this manner would neither eclipse any atom of `A` layer nor any atom of `B` layer. Thus, it would be a unique layer, referred as `C` layer and the layering pattern generated is `ABCABC` ....... known as cubic close packed (CCP) structure, which corresponds to face centred cubic (FCC) unit cell.