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Selective merohedry

In twinning by merohedry, the twin operations belong to the point group of the crystal lattice. When twinning concerns an OD structure, the definition of merohedry should take into account not only the point group of the crystal and that of the lattice, but also the point group of the family structure. If the latter is a subgroup of the twin point group, some of the twin laws restore only a part of the reciprocal lattice nodes corresponding to the family reflections (the reflections common to all the OD structures of a family). This kind of twinning is called twinning by selective merohedry: it can occur when the Laue point group of the crystal is lower than the twin point group, i.e. in the so-called “class II twins”.