9.8: 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”.