Explain the types of polymorphism

In terms of  thermodynamics, there are two kinds of polymorphism. For a monotropic system, a plot of the free energy of the various polymorphs against temperature do not cross before all polymorphs melt--in other words, any transition from one polymorph to another will be irreversible. For an enantiotropy system, a plot of the free energy against temperature shows a crossing point before the various melting points, and it might be possible to convert reversibly among the two polymorphs on heating and cooling.