The ability of a substance to crystallize with more than one type of structure is known as polymorphism, and a particular form is known as a polymorph. Four mechanisms are possible: displacive, reconstitutive, order-disorder, and polytropism. A displacive polymorphism is the internal arrangement changes by a small energy and no bonds are broken. The process is therefore reversible. A reconstitutive polymorphism is the breaking of atomic bonds and reassembly in a different structure. This requires a large change in energy. An order-disorder polymorphism is common in alloys. Polytropism occurs when to polymorphs differ only in the stacking of identical 2-D sheets (as in hexagonal and cubic close packing, for example).