Close-packed layers: each atom touches 6 others (hexagonal arrangement). HCP: ABAB stacking (atoms in 3rd layer directly above 1st). CCP (= FCC): ABCABC stacking (3rd layer offset from both 1st and 2nd). Both give CN = 12 and 74.05% packing efficiency. Between two close-packed layers: tetrahedral voids (apex up and down, CN 4) and octahedral voids (CN 6). For n atoms: tetrahedral voids = 2n, octahedral voids = n. Total voids = 3n.