Pseudo extra reflection conditions

The extra reflection conditions as given in Volume A of International Tables for Crystallography may not be rigorously fulfilled if the thermal vibrations and positional or static disorder of the atoms are not isotropic. Although small in most cases, the intensities of reflections violating the extra reflection conditions can become measurable for large thermal and positional anisotropy. It would be useful to have an inventory of these `pseudo extra reflection conditions' for all space groups in the early stage of structure investigation. The subject is demonstrated with the aid of four significant examples and the possibility of predicting without calculation the status of the largest part of the extra reflection conditions is discussed.