Piezoelectricity is a property of certain classes of crystalline materials. This piezoelectric phenomenon is similar to electrostriction, which is a property of all dielectrics. They have different mechanisms when subjected to an electric field. Usually the electrostrictive effect is very small and the direction of this small change in geometry does not change if the direction of electrical field is reversed. In contrast to this situation, piezoelectric materials exhibit a reversal in the direction of geometrical change when the direction of the electrical field is reversed.