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Introduction

The Ad-hoc Committee appointed in 1980 to consider `nomenclature problems concerning symmetry operations and symmetry elements in space groups' has issued two Reports entitled Nomenclature for Crystal Families, Bravais-Lattice Types and Arithmetic Classes (de Wolff et al., 1985) and Definition of Symmetry Elements in Space Groups and Point Groups (de Wolff et al., 1989). As noted in the 1989 Report, the only outstanding problem concerning symmetry operations is that of choosing appropriate symbols, since the concept is clear. A provisional notation has been adopted in International Tables for Crystallography (1983, 1989), referred to hereafter as ITA83.

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