The articles published in this Special Issue reprint offer a sound update of research fields

where the concepts of modular crystallography are important and provide unique keys to

understand and to solve problems of structural crystallography. Polytypism,

polysomatism, and twinning are fertile fields of research, and their basic principles-

o����en coupled with the OD (order-disorder) theory-are powerful tools to solve and

classify related crystal structures. Research on twinning and its consequences on

structure and properties of crystalline materials is cutting-edge, e.g., dealing with

relations between twin walls and piezoelectricity.