International
Tables for Crystallography Volume A Spacegroup symmetry Edited by Th. Hahn © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. A, Preface.
Preface^{a}Institut für Kristallographie, RheinischWestfälische Technische Hochschule, D52056 Aachen, Germany 
The present work can be considered as the first volume of the third series of the International Tables. The first series was published in 1935 in two volumes under the title Internationale Tabellen zur Bestimmung von Kristallstrukturen with C. Hermann as editor. The publication of the second series under the title International Tables for Xray Crystallography started with Volume I in 1952, with N. F. M. Henry and K. Lonsdale as editors. [Full references are given at the end of Part 2. Throughout this volume, the earlier editions are abbreviated as IT (1935) and IT (1952).] Three further volumes followed in 1959, 1962 and 1974. Comparison of the title of the present series, International Tables for Crystallography, with those of the earlier series reveals the progressively more general nature of the tables, away from the special topic of Xray structure determination. Indeed, it is the aim of the present work to provide data and text which are useful for all aspects of crystallography.
The present volume is called A in order to distinguish it from the numbering of the previous series. It deals with crystallographic symmetry in `direct space'. There are six other volumes in the present series: A1 (Symmetry relations between space groups), B (Reciprocal space), C (Mathematical, physical and chemical tables), D (Physical properties of crystals), E (Subperiodic groups) and F (Crystallography of biological macromolecules).
The work on this series started at the Rome Congress in 1963 when a new `Commission on International Tables' was formed, with N. F. M. Henry as chairman. The main task of this commission was to prepare and publish a Pilot Issue, consisting of five parts as follows:

The Pilot Issue was widely distributed with the aim of trying out the new ideas on the crystallographic community. Indeed, the responses to the Pilot Issue were a significant factor in determining the content and arrangement of the present volume.
Active preparation of Volume A started at the Kyoto Congress in 1972 with a revised Commission under the Chairmanship of Th. Hahn. The main decisions on the new volume were taken at a full Commission meeting in August 1973 at St. Nizier, France, and later at several smaller meetings at Amsterdam (1975), Warsaw (1978) and Aachen (1977/78/79). The manuscript of the volume was essentially completed by the time of the Ottawa Congress (1981), when the tenure of the Commission officially expired.
The major work of the preparation of the spacegroup tables in the First Edition of Volume A was carried out between 1972 and 1978 by D. S. Fokkema at the Rekencentrum of the Rijksuniversiteit Groningen as part of the Computer trial project, in close cooperation with A. Vos, D. W. Smits, the Editor and other Commission members. The work developed through various stages until at the end of 1978 the complete planegroup and spacegroup tables were available in printed form. The following years were spent with several rounds of proofreading of these tables by all members of the editorial team, with preparation and many critical readings of the various theoretical sections and with technical preparations for the actual production of the volume.
The First Edition of Volume A was published in 1983. With increasing numbers of later `Revised Editions', however, it became apparent that corrections and modifications could not be done further by `cutandpaste' work based on the printed version of the volume. Hence, for this Fifth Edition, the plane and spacegroup data have been reprogrammed and converted to an electronic form by M. I. Aroyo and P. B. Konstantinov (details are given in the following article Computer Production of Volume A ) and the text sections have been rekeyed in SGML format. The production of the Fifth Edition was thus completely computerbased, which should allow for easier corrections and modifications in the future, as well as the possibility of an electronic version of the volume.
The present volume treats the symmetries of one, two and threedimensional space groups and point groups in direct space. It thus corresponds to Volume 1 of IT (1935) and to Volume I of IT (1952). Not included in Volume A are `partially periodic groups', like layer, rod and ribbon groups, or groups in dimensions higher than three. (Subperiodic groups are discussed in Volume E of this series.) The treatment is restricted to `classical' crystallographic groups (groups of rigid motions); all extensions to `generalized symmetry', like antisymmetric groups, colour groups, symmetries of defect crystals etc., are beyond the scope of this volume.
Compared to its predecessors, the present volume is considerably increased in size. There are three reasons for this:
The new features of the description of each space group, as compared to IT (1952), are as follows:
The volume falls into two parts which differ in content and, in particular, in the level of approach:
At the end of each part, references are given for further studies.
The crystallographic calculations and the computer typesetting procedures for the First Edition (1983) were performed by D. S. Fokkema. For the Fifth Edition, the spacegroup data were reprogrammed and converted to an electronic form by M. I. Aroyo and P. B. Konstantinov. Details are given in the following article Computer Production of Volume A .
The following authors supplied lists of data for the spacegroup tables in Parts 6 and 7 :
The spacegroup diagrams for the First Edition were prepared as follows:
New diagrams for all 17 plane groups and all 230 space groups were incorporated in stages in the Second, Third and Fourth Editions of this volume. This project was carried out at Aachen by R. A. Becker. All data and diagrams were checked by at least two further members of the editorial team until no more discrepancies were found.
At the conclusion of this Preface, it should be mentioned that during the preparation of this volume several problems led to long and sometimes controversial discussions. One such topic was the subdivision of the hexagonal crystal family into either hexagonal and trigonal or hexagonal and rhombohedral systems. This was resolved in favour of the hexagonal–trigonal treatment, in order to preserve continuity with IT (1952); the alternatives are laid out in Sections 2.1.2 and 8.2.8 .
An even greater controversy evolved over the treatment of the monoclinic space groups and in particular over the question whether the b axis, the c axis, or both should be permitted as the `unique' axis. This was resolved by the Union's Executive Committee in 1977 by taking recourse to the decision of the 1951 General Assembly at Stockholm [cf. Acta Cryst. (1951). 4, 569 ]. It is hoped that the treatment of monoclinic space groups in this volume (cf. Section 2.2.16 ) represents a compromise acceptable to all parties concerned.