Pressure dependence of versus T curves

Clearly there is some pressure at which the three curves intersect at the same temperature. This temperature and pressure define the triple point all three phases coexist in equilibrium at the triple point. 

Whether or not a particular material will sublime under reduced pressure rather than melt depends entirely on the individual properties of the substance. Water, for example, sublimes at pressures below 611 Pa. The higher the melting point, and the smaller the difference between the melting point and boiling point at 1 atm pressure, the higher will be the pressure below which sublimation is observed. The pressure (in atm) below which sublimation is observed can be estimated for substances obeying Trouton s rule by the formula 

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