When a third layer of 2D hexagonal close packed layer is placed over the second layer, there are two possibilities.

First possibility is Covering the Tetrahedral voids.
Place 2 layers of spheres such that the second layer fits in the depression of first layer. First layer is named as A and the second layer as B

Place a third layer of spheres such that it covers the tetrahedral voids.
When arranged in this way, spheres of third layer align exactly with the spheres of first layer.
So, the third layer can also be named as ‘A’.
Similarly, the fourth layer of spheres aligns with the spheres of second layer and can be named as ‘B’.
Thus the pattern of spheres is repeated in alternate layers.
This pattern can be named as ABAB…pattern.
And the structure is known as hexagonal close packed structure (hcp).
Example: metals like Magnesium, Zinc, cadmium have this kind of packing.

This type of packing is very efficient, because of the very less space between the spheres and the packing efficiency of this lattice is 74%.
Each sphere is in contact with 12 nearest atoms. So, the coordination number is 12.
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