Apr 012002

*J. Appl. Cryst.* (2002), **35**, 168-174. Â Â [ doi:10.1107/S0021889802000249 ]

Although non-merohedrally twinned crystal structures can normally be solved without difficulty, problems usually arise during refinement. Careful analysis of poorly fitting data reveals that they belong predominantly to certain distinct zones in which |*F*_{o}|^{2} is systematically larger than |*F*_{c}|^{2}. In the computer program *ROTAX*, a set of data with the largest values of (|*F*_{o}^{2}| – |*F*_{c}^{2}|)/*u*(|*F*_{o}^{2}|) is identified and their indices transformed by rotations or roto-inversions about possible direct- and reciprocal-lattice directions. Matrices that transform the indices of the poorly fitting data to integers are identified as possible twin laws.