Refinement of Twinned Structures with <i>SHELXL</i>97

Four examples of the refinement of twinned structures are discussed. These structures illustrate three different types of twins: twinning by merohedry, pseudo-merohedry and non-merohedral twins. How the twinning was detected, how the structures were solved and how they were refined are shown in this paper. The difference between a refinement as a disordered model and a twinned model is illustrated. Sometimes the twin law was necessary to solve the structure, while in the other examples the twinning was first recognized during refinement of the structures. Finally, a list of `characteristic warning signs' is presented which are indicative of possible twinning.