A phase diagram shows the preferred physical states of matter at different temperatures and pressure. Within each phase the material is uniform with respect to its chemical composition and physical state. At typical room temperatures and pressure (shown as an 'x' below) water is a liquid, but it becomes solid (i.e. ice) if its temperature is lowered below 273 K and gaseous (i.e. steam) if its temperature is raised above 373 K, at the same pressure. Each line represents a phase boundary and gives the conditions when two phases coexist. Here, a change in temperature or pressure may cause the phases to abruptly change from one to the other. Where three lines join, there is a 'triple point' when three phases coexist but may abruptly and totally change into each other given a change in temperature or pressure. Four lines cannot meet at a single point. A 'critical point' is where the properties of two phases become indistinguishable from each other. The phase diagram of water is complex,g having a number of triple points and one or possibly two critical points. Many of the crystalline forms may remain metastable in much of the low-temperature phase space at lower pressures.