Contribution by position: corner = 1/8 (shared by 8 cells), edge = 1/4 (shared by 4), face = 1/2 (shared by 2), body centre = 1 (unshared). SC: 8 corners x 1/8 = 1. BCC: 8$\frac{1}{8}$ + 1(body) = 2. FCC: 8$\frac{1}{8}$ + 6$\frac{1}{2}$ = 4. These Z values are used in the density formula d = $\frac{ZM}{N_A a^3}$.