AbstractThe geometric theory of twinning was developed almost a century ago. Despite its age, it still represents the fundamental approach to the analysis and interpretation of twinned crystals, in both the direct and the reciprocal space. In recent years, this theory has been extended not only in its formalism (group-subgroup analysis, chromatic symmetry) but also in its classification of special cases that were not recognized before. The geometrical theory of twinning is thus reviewed here

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... h emphasis on lattice aspects and recent developments. The classification of various types of twins starts with Friedel's well-known scheme, which distinguishes four cases according to whether the twin index