 Some solids with a high formula_1 form density waves while others choose a superconducting or magnetic ground state at low temperatures, because of the existence of nesting vectors in the materials' Fermi surfaces.The concept of a nesting vector is illustrated in the Figure for the famous case of chromium, which transitions from a paramagnetic to SDW state at a Néel temperature of 311 K. Cr is a body-centered cubic metal whose Fermi surface features many parallel boundaries between electron pockets centered at formula_6 and hole pockets at H. These large parallel regions can be spanned by the nesting wavevector formula_4 shown in red.The real-space periodicity of the resulting spin-density wave is given by formula_8.