The temperature at which a solid substance undergoes fusion, that is, melts, changes from solid to liquid form. The melting point of a substance should be considered a property of its crystalline form only. At the melting point the liquid and solid forms of a substance exist in equilibrium. All substances of crystalline nature have their characteristic melting points. For very pure substances the temperature range over which the process of fusion occurs is very small. The melting point of a pure crystalline solid is a function of pressure; it increases with increasing pressure for most substances. However, in the case of ice (and a few other substances) the melting point decreases with increasing pressure (see regelation). Under a pressure of one standard atmosphere, the melting point of pure ice is the same as the ice point, that is, 0C.