Explain three-dimensional close packing from two-dimensional square close-packing. How is the primitive cubic unit cell obtained?

(N/A) Three-dimensional structures are obtained by stacking two-dimensional layers one above the other.
$A$ simple cubic lattice is formed by an $AAA \dots$ arrangement.
In this type of arrangement,the second square close-packed layer is placed exactly above the first square close-packed layer such that the spheres of the first and second layers are in the same alignment horizontally and vertically. In a similar way,more layers can be placed one above the other.
If the arrangement of spheres in the first layer is called $A$ type,then all the layers have the same arrangement. Thus,this lattice has an $AAAAA \dots$ type arrangement,and the lattice generated is a simple cubic unit cell or primitive cubic unit cell. The coordination number of each sphere is $6$.