Liquid crystals are substances which exhibit thermodynamic phases intermediate between crystalline solid and ordinary liquid, called mesophases, which are distinct from both ordinary liquid and crystalline solid. They have the property of fluidity, that characterizes the liquids, and at the same time exhibit the phenomenon of birefringence, as happens to crystalline solids. Microscopically, the liquid crystals are distinguished by displaying only one of two types of ordering that characterize the molecular crystalline solids. In a molecular crystal, the positions of the centers of the molecules form a spatial ordered structure and in addition the molecules are ordered as to the orientation. In an ordinary liquid, both positional and orientational orders are absent. In a liquid crystal molecules have orientational order but positional order is absent. In the opposite situation to liquid crystals, the molecules of a plastic crystal exhibit positional order but orientational order is absent.