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Any crystal whose point group is a subgroup of index p of its lattice symmetry (merohedral crystals, p = 2, 4 or 8) can be referred to p non-equivalent possible settings of the cordinate axes. These possible settings are tabulated, for all 44 oriented point groups and for each setting s, as the triplet of indices hsksls, into which the original hkl transforms, together with the relevant transformation operation. At least one such set of suitable observed structure factors should be published with every structure description for merohedral crystals so that the chosen setting can be identified. A list of the types of reflections unsuitable for orientation purposes is given. A unique orientation of the coordinate axes could be reached experimentally, without structural knowledge, by attributing to the largest Iobs, indices hkl in a specified asymmetric domain of the point-group symmetry of the lattice. The table also serves as a complete collection of possible twin laws in twinning by merohedry; each transformed symbol hsksls represents the reflection which, on the diffraction pattern of the twin, contributes its intensity to that of hkl.
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