This single layer is the precursor of the body centered cubic (BCC) unit cell. The lattice points are in the same general form as the simple cubic (SC) unit cell, with the exception that the atoms are ...
occupying the octahedral holes. Notice that the octahedral holes are along the edges (for a net of three cations) and in the center of the cube (one more cation). Thus we have four anions (FCC) and ...