The positions of the layers in a body-centered cubic structure alternate. The third layer sits directly above the first layer. The fourth layer sits directly above the second layer and the structure continues with alternating layers.

Click on a site in the figure where a sphere from the next layer of spheres will sit.

We have used different colored spheres to identify the various layers in the BCC structure. However, don't forget that all of the atoms in a metal really are identical.