Acta Cryst. (1998). B54, 443±449 Re®nement of Twinned Structures with SHELXL97

Four examples of the re®nement of twinned structures are discussed. These structures illustrate three different types of twins: twinning by merohedry, pseudo-mero-hedry and non-merohedral twins. How the twinning was detected, how the structures were solved and how they were re®ned are shown in this paper. The difference between a re®nement 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 ®rst recognized during re®nement of the structures. Finally, a list of `character-istic warning signs ' is presented which are indicative of possible twinning. 1