International
Tables for Crystallography Volume A Spacegroup symmetry Edited by Th. Hahn © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. A, ch. 2.2, pp. 2024
Section 2.2.6. Spacegroup diagrams^{a}Institut für Kristallographie, RheinischWestfälische Technische Hochschule, Aachen, Germany, and ^{b}Laboratorium voor Chemische Fysica, Rijksuniversiteit Groningen, The Netherlands 
The spacegroup diagrams serve two purposes: (i) to show the relative locations and orientations of the symmetry elements and (ii) to illustrate the arrangement of a set of symmetrically equivalent points of the general position.
All diagrams are orthogonal projections, i.e. the projection direction is perpendicular to the plane of the figure. Apart from the descriptions of the rhombohedral space groups with `rhombohedral axes' (cf. Section 2.2.6.6), the projection direction is always a cell axis. If other axes are not parallel to the plane of the figure, they are indicated by the subscript p, as or . This applies to one or two axes for triclinic and monoclinic space groups (cf. Figs. 2.2.6.1 to 2.2.6.3), as well as to the three rhombohedral axes in Fig. 2.2.6.9.
The graphical symbols for symmetry elements, as used in the drawings, are displayed in Chapter 1.4 .
In the diagrams, `heights' h above the projection plane are indicated for symmetry planes and symmetry axes parallel to the projection plane, as well as for centres of symmetry. The heights are given as fractions of the shortest lattice translation normal to the projection plane and, if different from 0, are printed next to the graphical symbols. Each symmetry element at height h is accompanied by another symmetry element of the same type at height (this does not apply to the horizontal fourfold axes in the cubic diagrams). In the spacegroup diagrams, only the symmetry element at height h is indicated (cf. Chapter 1.4 ).
Schematic representations of the diagrams, displaying the origin, the labels of the axes, and the projection direction [uvw], are given in Figs. 2.2.6.1 to 2.2.6.10 (except Fig. 2.2.6.6). The generalposition diagrams are indicated by the letter .
Each description of a plane group contains two diagrams, one for the symmetry elements (left) and one for the general position (right). The two axes are labelled a and b, with a pointing downwards and b running from left to right.
For each of the two triclinic space groups, three elevations (along a, b and c) are given, in addition to the generalposition diagram (projected along c) at the lower right of the set, as illustrated in Fig. 2.2.6.1.
The diagrams represent a reduced cell of type II for which the three interaxial angles are nonacute, i.e. . For a cell of type I, all angles are acute, i.e. . For a discussion of the two types of reduced cells, reference is made to Section 9.2.2 .
The `complete treatment' of each of the two settings contains four diagrams (Figs. 2.2.6.2 and 2.2.6.3). Three of them are projections of the symmetry elements, taken along the unique axis (upper left) and along the other two axes (lower left and upper right). For the general position, only the projection along the unique axis is given (lower right).
The `synoptic descriptions' of the three cell choices (for each setting) are headed by a pair of diagrams, as illustrated in Fig. 2.2.6.4. The drawings on the left display the symmetry elements and the ones on the right the general position (labelled ). Each diagram is a projection of four neighbouring unit cells along the unique axis. It contains the outlines of the three cell choices drawn as heavy lines. For the labelling of the axes, see Fig. 2.2.6.4. The headline of the description of each cell choice contains a smallscale drawing, indicating the basis vectors and the cell that apply to that description.
The spacegroup tables contain a set of four diagrams for each orthorhombic space group. The set consists of three projections of the symmetry elements [along the c axis (upper left), the a axis (lower left) and the b axis (upper right)] in addition to the generalposition diagram, which is given only in the projection along c (lower right). The projected axes, the origins and the projection directions of these diagrams are illustrated in Fig. 2.2.6.5. They refer to the socalled `standard setting' of the space group, i.e. the setting described in the spacegroup tables and indicated by the `standard Hermann–Mauguin symbol' in the headline.

Orthorhombic space groups. Diagrams for the `standard setting' as described in the spacegroup tables ( = generalposition diagram). 
For each orthorhombic space group, six settings exist, i.e. six different ways of assigning the labels a, b, c to the three orthorhombic symmetry directions; thus the shape and orientation of the cell are the same for each setting. These settings correspond to the six permutations of the labels of the axes (including the identity permutation); cf. Section 2.2.16: The symbol for each setting, here called `setting symbol', is a shorthand notation for the transformation of the basis vectors of the standard setting, a, b, c, into those of the setting considered. For instance, the setting symbol cab stands for the cyclic permutation or where a′, b′, c′ is the new set of basis vectors. An interchange of two axes reverses the handedness of the coordinate system; in order to keep the system righthanded, each interchange is accompanied by the reversal of the sense of one axis, i.e. by an element in the transformation matrix. Thus, denotes the transformation The six orthorhombic settings correspond to six Hermann–Mauguin symbols which, however, need not all be different; cf. Table 2.2.6.1.^{2}

In the earlier (1935 and 1952) editions of International Tables, only one setting was illustrated, in a projection along c, so that it was usual to consider it as the `standard setting' and to accept its cell edges as crystal axes and its spacegroup symbol as `standard Hermann–Mauguin symbol'. In the present edition, however, all six orthorhombic settings are illustrated, as explained below.
The three projections of the symmetry elements can be interpreted in two ways. First, in the sense indicated above, that is, as different projections of a single (standard) setting of the space group, with the projected basis vectors a, b, c labelled as in Fig. 2.2.6.5. Second, each one of the three diagrams can be considered as the projection along c′ of either one of two different settings: one setting in which b′ is horizontal and one in which b′ is vertical (a′, b′, c′ refer to the setting under consideration). This second interpretation is used to illustrate in the same figure the spacegroup symbols corresponding to these two settings. In order to view these projections in conventional orientation (b′ horizontal, a′ vertical, origin in the upper left corner, projection down the positive c′ axis), the setting with b′ horizontal can be inspected directly with the figure upright; hence, the corresponding spacegroup symbol is printed above the projection. The other setting with b′ vertical and a′ horizontal, however, requires turning the figure over 90°, or looking at it from the side; thus, the spacegroup symbol is printed at the left, and it runs upwards.
The `setting symbols' for the six settings are attached to the three diagrams of Fig. 2.2.6.6, which correspond to those of Fig. 2.2.6.5. In the orientation of the diagram where the setting symbol is read in the usual way, a′ is vertical pointing downwards, b′ is horizontal pointing to the right, and c′ is pointing upwards from the page. Each setting symbol is printed in the position that in the spacegroup tables is actually occupied by the corresponding full Hermann–Mauguin symbol. The changes in the spacegroup symbol that are associated with a particular setting symbol can easily be deduced by comparing Fig. 2.2.6.6 with the diagrams for the space group under consideration.
Not all of the 59 orthorhombic space groups have all six projections distinct, i.e. have different Hermann–Mauguin symbols for the six settings. This aspect is treated in Table 2.2.6.1. Only 22 space groups have six, 25 have three, 2 have two different symbols, while 10 have all symbols the same. This information can be of help in the early stages of a crystalstructure analysis.
The six setting symbols listed in the second paragraph of this section form the column headings of the orthorhombic entries in Table 4.3.2.1 , which contains the extended Hermann–Mauguin symbols for the six settings of each orthorhombic space group. Note that some of these setting symbols exhibit different sign changes compared with those in Fig. 2.2.6.6.
The pairs of diagrams for these space groups are similar to those in IT (1935) and IT (1952). Each pair consists of a generalposition diagram (right) and a diagram of the symmetry elements (left), both projected along c, as illustrated in Figs. 2.2.6.7 and 2.2.6.8.
The seven rhombohedral R space groups are treated in two versions, the first based on `hexagonal axes' (obverse setting), the second on `rhombohedral axes' (cf. Sections 2.1.3 and 2.2.2). The pairs of diagrams are similar to those in IT (1952); the left or top one displays the symmetry elements, the right or bottom one the general position. This is illustrated in Fig. 2.2.6.9, which gives the axes a and b of the triple hexagonal cell and the projections of the axes of the primitive rhombohedral cell, labelled and . For convenience, all `heights' in the spacegroup diagrams are fractions of the hexagonal c axis. For `hexagonal axes', the projection direction is [001], for `rhombohedral axes' it is [111]. In the generalposition diagrams, the circles drawn in heavier lines represent atoms that lie within the primitive rhombohedral cell (provided the symbol `−' is read as rather than as ).
The pairs of drawings for the hexagonal and the rhombohedral descriptions of a space group are the same. In the rhombohedral descriptions of space groups Nos. 166 and 167, and , the diagrams are omitted for reasons of space, and the reader is referred to the drawings in the hexagonal descriptions.
For each cubic space group, one projection of the symmetry elements along [001] is given, Fig. 2.2.6.10; for details of the diagrams, see Chapter 1.4 and Buerger (1956). For facecentred lattices F, only a quarter of the unit cell is shown; this is sufficient since the projected arrangement of the symmetry elements is translationequivalent in the four quarters of an F cell. The three stereoscopic generalposition diagrams in the lower part of the page are explained below.
The cubic diagrams given in IT (1935) were quite different from the ones used here. No drawings for cubic space groups were provided in IT (1952).
Readers who wish to compare other approaches to spacegroup diagrams and their history are referred to IT (1935), IT (1952) and the following publications: Astbury & Yardley (1924); Belov et al. (1980); Buerger (1956); Fedorov (1895; English translation, 1971); Friedel (1926); Hilton (1903); Niggli (1919); Schiebold (1929).
References
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Astbury, W. T. & Yardley, K. (1924). Tabulated data for the examination of the 230 space groups by homogeneous Xrays. Philos. Trans. R. Soc. London Ser. A, 224, 221–257.
Belov, N. V., Zagal'skaja, Ju. G., Litvinskaja, G. P. & EgorovTismenko, Ju. K. (1980). Atlas of the space groups of the cubic system. Moscow: Nauka. (In Russian.)
Buerger, M. J. (1956). Elementary crystallography. New York: Wiley.
Fedorov, E. S. (1895). Theorie der Kristallstruktur. Einleitung. Regelmässige Punktsysteme (mit übersichtlicher graphischer Darstellung). Z. Kristallogr. 24, 209–252, Tafel V, VI. [English translation by D. & K. Harker (1971). Symmetry of crystals, esp. pp. 206–213. Am. Crystallogr. Assoc., ACA Monograph No. 7.]
Friedel, G. (1926). Leçons de cristallographie. Nancy/Paris/Strasbourg: BergerLevrault. [Reprinted: Paris: Blanchard (1964).]
Hilton, H. (1903). Mathematical crystallography. Oxford: Clarendon Press. [Reprint: New York: Dover (1963).]
Langlet, G. A. (1972). FIGATOM: a new graphic program for stereographic crystal structure illustrations. J. Appl. Cryst. 5, 66–71.
Niggli, P. (1919). Geometrische Kristallographie des Diskontinuums. Leipzig: Borntraeger. [Reprint: Wiesbaden: Sändig (1973).]
Schiebold, E. (1929). Über eine neue Herleitung und Nomenklatur der 230 kristallographischen Raumgruppen mit Atlas der 230 RaumgruppenProjektionen. Text, Atlas. In Abhandlungen der MathematischPhysikalischen Klasse der Sächsischen Akademie der Wissenschaften, Band 40, Heft 5. Leipzig: Hirzel.