Hexagonal Close Packed (HCP) Structure:-

 The cell of HCP system has 12 corner atoms, one at each corner of the hexagon with two base centered atoms, one at the top face of the hexagon and another at the bottom face of the hexagon as shown in figure.

 In addition to the corner and base atoms, there are three symmetrically arranged atoms in among the top and bottom face of the hexagon. Three additional atoms are located among top and bottom planes, which are demonstrated in hard sphere model as shown in figure