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(1/8) + 1(body) = 2. FCC: 8(1/8) + 6(1/2) = 4. These Z values are used in the density formula d = ZM/(N_A a^3).