]In terms of thermodynamics, two types of polymorphic behaviour are recognized. For a monotropic system, plots of the free energies of the various polymorphs against temperature do not cross before all polymorphs melt. As a result, any transition from one polymorph to another below the melting point will be irreversible. For an enantiotropic system, a plot of the free energy against temperature shows a crossing point before the various melting points. It may also be possible to convert interchangeably between the two polymorphs by heating or cooling, or through physical contact with a lower energy polymorph.
A simple model of polymorphism is to model the Gibbs free energy of a ball-shaped crystal as . Here, the first term  is the surface energy, and the second term  is the volume energy. Both parameters . The function  rises to a maximum before dropping, crossing zero at . In order to crystallize, a ball of crystal much overcome the energetic barrier to the  part of the energy landscape.
Now, suppose there are two kinds of crystals, with different energies  and , and if they have the same shape as in Figure 2, then the two curves intersect at some . Then the system has three phases:
* . Crystals tend to dissolve. Amorphous phase.
* . Crystals tend to grow as form 1.
* . Crystals tend to grow as form 2.
If the crystal is grown slowly, it could be kinetically stuck in form 1.