All 2D crystal structures fall into one of 5 Bravais lattice types. The direct lattice (left) shows atomic positions; the reciprocal lattice (right) is its Fourier dual, defining Brillouin zones and diffraction patterns. b₁·a₂ = b₂·a₁ = 0, b₁·a₁ = b₂·a₂ = 2π.