research papers
The effect of merohedric twinning on the reflection conditions and the symmetry of the diffraction pattern is analysed systematically and criteria to confirm or exclude the presence of twinning are presented.
book reviews
editorial
The current activities of the IUCr Commission on Mathematical Crystallography are described and the articles in this virtual special issue on mathematical crystallography are introduced.
research papers
The relative frequency of occurrence of staurolite twins is explained by the quasi-restoration of the cationic substructures in terms of the pseudo-eigensymmetry of the corresponding crystallographic orbits.