In twinning by merohedry, the twin operations belong to the crystal lattice point group. When twinning concerns an OD structure, i.e. a polytype in which layer pairs are geometrically equivalent, 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 point group of the twin, 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). For this kind of twinning the definition of selective merohedry is introduced: 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”. Detection of selective merohedry from the diffraction pattern is discussed.