In the previous Chapters 5 and 6, we have assumed the plate material to be isotropic, which means that the material properties at a point are the same in all directions. However, certain materials have properties that are not independent of the direction. These materials are said to be anisotropic. Examples of anisotropic materials are two-way reinforced concrete slabs, plywood, and fiber-reinforced plastics. Structural anisotropy is also introduced by means of ribs or corrugations. Consequently, to obtain a reasonable agreement between analysis and the actual behavior, it is necessary to consider the anisotropy of such plates in the calculations.