The direct (the left plot) and the reciprocal lattice (the right plot) for an fcc(111) surface. In the direct lattice γ is the skewing angle, and a1 and a2 are the primitive vectors that span the surface unit cell. Miller indices are shown in the reciprocal lattice to indicate the different diffraction channels. Red hexagon shows the 2D Wigner–Seitz cell. The concentric hexagons indicate how the diffraction order is defined for the (111) lattice. The <10 ̄1> and <11 ̄2> directions have been indicated in both figures in green.