The document discusses close packing structures in crystals. It defines hexagonal close packing and cubic close packing, which differ based on how successive layers are stacked. Cubic close packing is shown to be equivalent to a face-centered cubic unit cell. Close packing yields a packing fraction of 0.74, maximizing the efficient use of space, while primitive unit cells have significantly lower packing fractions with more empty space. The key differences between close packing structures and their implications are summarized.