See jmol: Cubic Closest Packed
	See jmol: Body Centered Cubic

 

When substances form solids they most often do so in specific patterns or crystal structures. Crystals are typically described by their smallest repeating unit, termed the unit cell. The figure above illustrates the positions for a face-centered cubic unit cell. In most browsers, clicking the link on the left of the table will bring up the 'jmol' applet from which you may manipulate a model of the face centered cubic structure.

Geometry limits the number of crystal structures. That is, there are only 230 so-called space groups. These permit 14 Bravais lattices and 6 (or 7) crystal systems.

Purdue chemistry site for reference information.