Different states of matter are distinguished by their internal structures or orders. For many years, all the orders were thought to be associated with symmetries (or rather, the breaking of symmetries). For example, when a gas changes into a crystal, the continuous translation symmetry of the gas is reduced to a discrete translation symmetry of the crystal. Based on the relation between orders and symmetries, a general theory of ordering was developed by L. Landau. The success of Landau's theory gave rise to a widespread belief that, at least in principle, all the kinds of order that matter can display were understood.