International
Tables for Crystallography Volume A Spacegroup symmetry Edited by M. I. Aroyo © International Union of Crystallography 2016 
International Tables for Crystallography (2016). Vol. A, ch. 2.1, pp. 142144

A plane group or space group usually is described by means of a crystallographic coordinate system, consisting of a crystallographic basis (basis vectors are lattice vectors) and a crystallographic origin (origin at a centre of symmetry or at a point of high site symmetry). The choice of such a coordinate system is not mandatory, since in principle a crystal structure can be referred to any coordinate system; cf. Chapters 1.3 and 1.5 .
The selection of a crystallographic coordinate system is not unique. Conventionally, a righthanded set of basis vectors is taken such that the symmetry of the plane or space group is displayed best. With this convention, which is followed in the present volume, the specific restrictions imposed on the cell parameters by each crystal family become particularly simple. They are listed in columns 6 and 7 of Table 2.1.1.1. If within these restrictions the smallest cell is chosen, a conventional (crystallographic) basis results. Together with the selection of an appropriate conventional (crystallographic) origin (cf. Sections 2.1.3.2 and 2.1.3.7), such a basis defines a conventional (crystallographic) coordinate system and a conventional cell. The conventional cell of a point lattice or a space group, obtained in this way, turns out to be either primitive or to exhibit one of the centring types listed in Table 2.1.1.2. The centring type of a conventional cell is transferred to the lattice which is described by this cell; hence, we speak of primitive, facecentred, bodycentred etc. lattices. Similarly, the cell parameters are often called lattice parameters; cf. Chapters 1.3 and 3.1 for further details.
In the triclinic, monoclinic and orthorhombic crystal systems, additional conventions (for instance cell reduction or metrical conventions based on the lengths of the cell edges) are needed to determine the choice and the labelling of the axes. Reduced bases are treated in Chapter 3.1 , orthorhombic settings in Section 2.1.3.6, and monoclinic settings and cell choices in Section 2.1.3.15 (cf. Section 1.5.4 for a detailed treatment of alternative settings of space groups).
In this volume, all space groups within a crystal family are referred to the same kind of conventional coordinate system, with the exception of the hexagonal crystal family in three dimensions. Here, two kinds of coordinate systems are used, the hexagonal and the rhombohedral systems. In accordance with common crystallographic practice, all space groups based on the hexagonal Bravais lattice hP (18 trigonal and 27 hexagonal space groups) are described only with a hexagonal coordinate system (primitive cell), whereas the seven space groups based on the rhombohedral Bravais lattice hR (the socalled `rhombohedral space groups', cf. Section 1.4.1 ) are treated in two versions, one referred to `hexagonal axes' (triple obverse cell) and one to `rhombohedral axes' (primitive cell); cf. Table 2.1.1.2. In practice, hexagonal axes are preferred because they are easier to visualize.
Table 2.1.1.2 contains only those conventional centring symbols which occur in the Hermann–Mauguin spacegroup symbols. There exist, of course, further kinds of centred cells which are unconventional, see for example the synoptic tables of plane (Table 1.5.4.3 ) and space (Table 1.5.4.4 ) groups discussed in Chapter 1.5 . The centring type of a cell may change with a change of the basis vectors; in particular, a primitive cell may become a centred cell and vice versa. Examples of relevant transformation matrices are contained in Table 1.5.1.1 .