The CDW in electronic charge is accompanied by a periodic distortion – essentially a superlattice – of the atomic lattice. The metallic crystals look like thin shiny ribbons (e.g., quasi-1-D NbSe 3 crystals) or shiny flat sheets (e.g., quasi-2-D, 1T-TaS 2 crystals). The CDW's existence was first predicted in the 1930s by Rudolf Peierls. He argued that a 1-D metal would be unstable to the formation of energy gaps at the Fermi wavevectors ± k F, which reduce the energies of the filled electronic states at ± k F as compared to their original Fermi energy E F. The temperature below which such gaps form is known as the Peierls transition temperature, T P.