The following is from the Learner.Org Chemistry course https://www.learner.org/courses/chemistry/about/about.html
Once the gas laws were formulated, chemists could analyze how materials transitioned from one phase to another, and how temperature and pressure affected these changes.
In 1897, a British metallurgist named Sir William Chandler Roberts-Austen (1843–1902) produced what is widely regarded as an early form of a now-common tool in chemistry and related disciplines: the phase diagram.
Modern phase diagrams show relationships between different states of matter under various combinations of temperature and pressure.
A substance can exist in two different states at once—for example, as a liquid and a gas, with molecules cycling from one state to the other.
It is also possible for a material to be both solid and liquid, with both melting and freezing taking place at its edges, or to exist as a solid and a gas.
Phase diagrams show what forms a substance will take under given temperatures and pressure levels, and where these equilibrium lines (when equal numbers of molecules are changing form in both directions) are located. (Figure 2-11)
Amazing: See a flask of liquid cyclohexane brought to the brink of its triple-point:
suddenly it can boil and freeze at the same time.