The Wigner-Seitz cell is the region of space closer to a given lattice point than to any other — a Voronoi cell of the lattice. It is the unique primitive unit cell (volume = 1/density) that has the full point-group symmetry of the lattice. The construction bisects lines to nearest neighbors with perpendicular planes; the intersection of all half-spaces is the cell. In reciprocal space, the Wigner-Seitz cell of the reciprocal lattice is the first Brillouin zone — the fundamental domain for Bloch wavevectors k, governing electronic band structure, phonon dispersion, and all wave phenomena in crystals.