In merohedric twinning, the lattices of the individuals are perfectly overlapped and the presence of twinning is not easily detected from the diffraction pattern, especially in the case of inversion twinning (class I). In general, the investigator has to consider three possible structural models: a crystal with space-group type H and point group P, either untwinned (H model) or twinned through an operation t in vector space (t-H model), and an untwinned crystal with space group G whose point group P' is obtained as an extension of P through the twin operation t (G model). In 71 cases, consideration of the reflection conditions may directly rule out the G model; in seven other cases the reflection conditions suggest a space group which does not correspond to the extension of H by the twin operation and the structure solution or at least the refinement will fail. When the twin operation belongs to a different crystal family (class IIB twinning: the crystal has a specialized metric), the presence of twinning can often be recognized by the peculiar effect it has on the reflection conditions.
Keywords: diffraction symmetry; merohedric twinning; reflection conditions.