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. 3.1, pp. 4445

In Section 2.2.13 , it has been shown that `extinctions' (sets of reflections that are systematically absent) point to the presence of a centred cell or the presence of symmetry elements with glide or screw components. Reflection conditions and Laue class together are expressed by the Diffraction symbol, introduced by Buerger (1935, 1942, 1969); it consists of the Laueclass symbol, followed by the extinction symbol representing the observed reflection conditions. Donnay & Harker (1940) have used the concept of extinctions under the name of `morphological aspect' (or aspect for short) in their studies of crystal habit (cf. Crystal Data, 1972). Although the concept of aspect applies to diffraction as well as to morphology (Donnay & Kennard, 1964), for the present tables the expression `extinction symbol' has been chosen because of the morphological connotation of the word aspect.
The Extinction symbols are arranged as follows. First, a capital letter is given representing the centring type of the cell (Section 1.2.1 ). Thereafter, the reflection conditions for the successive symmetry directions are symbolized. Symmetry directions not having reflection conditions are represented by a dash. A symmetry direction with reflection conditions is represented by the symbol for the corresponding glide plane and/or screw axis. The symbols applied are the same as those used in the Hermann–Mauguin spacegroup symbols (Section 1.3.1 ). If a symmetry direction has more than one kind of glide plane, for the diffraction symbol the same letter is used as in the corresponding spacegroup symbol. An exception is made for some centred orthorhombic space groups where two glideplane symbols are given (between parentheses) for one of the symmetry directions, in order to stress the relation between the diffraction symbol and the symbols of the `possible space groups'. For the various orthorhombic settings, treated in Table 3.1.4.1, the top lines of the twoline spacegroup symbols in Table 4.3.2.1 are used. In the monoclinic system, dummy numbers `1' are inserted for two directions even though they are not symmetry directions, to bring out the differences between the diffraction symbols for the b, c and a settings.
Example
Laue class:
Reflection conditions: As there are both c and n glide planes perpendicular to b, the diffraction symbol may be given as or as . In analogy to the symbols of the possible space groups, C1c1 (9) and , the diffraction symbol is called .
For another cell choice, the reflection conditions are: For this second cell choice, the glide planes perpendicular to b are n and a. The diffraction symbol is given as , in analogy to the symbols A1n1 (9) and adopted for the possible space groups.
References
Crystal Data (1972). Vol. I, General Editors J. D. H. Donnay & H. M. Ondik, Supplement II, pp. S41–52. Washington: National Bureau of Standards.Buerger, M. J. (1935). The application of plane groups to the interpretation of Weissenberg photographs. Z. Kristallogr. 91, 255–289.
Buerger, M. J. (1942). Xray crystallography, Chap. 22. New York: Wiley.
Buerger, M. J. (1969). Diffraction symbols. Chap. 3 of Physics of the solid state, edited by S. Balakrishna, pp. 27–42. London: Academic Press.
Donnay, J. D. H. & Harker, D. (1940). Nouvelles tables d'extinctions pour les 230 groupes de recouvrements cristallographiques. Nat. Can. 67, 33–69, 160.
Donnay, J. D. H. & Kennard, O. (1964). Diffraction symbols. Acta Cryst. 17, 1337–1340.