Abstract
Abstract According to Carmelo Giacovazzo crystallographic ‘twins are regular aggregates consisting of individual crystals of the same species joined together in some definite mutual orientation’. Georges Friedel distinguished between merohedral twinning (a special case which is racemic twinning) pseudo-merohedral twinning, reticular merohedral twinning, and non-merohedral twinning. This chapter defines all four types of crystallographic twinning. It describes how twin laws can be derived, how twinning can be taken into consideration during data reduction, absorption correction, scaling, and merging of the data; and how HKLF5 format files can be generated. Several programs are described in addition to SHELXL, and the typical warning signs for twinning are listed and twinning tests are explained. Six detailed examples are used to demonstrate how twin refinement is performed in SHELXL.
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