What's in a Name?

Sidney Abrahams and a Rose at the Bordeaux Congress in 1990. (Photo WLD)

This well known line from Act II, Scene 2 of Shakespeare's Romeo and Juliet, which continues "That which we call a rose by any other name would smell as sweet", is surely more applicable to Romeo than to crystallographic quantities. In crystallography, the use of multiple names for the same quantity, or the same name for different quantities, leads only to confusion in the literature. All international scientific unions are concerned when this kind of confusion arises in their fields of interest and many have established groups to guide authors toward the use of unambiguous nomenclature. The International Union of Crystallography  agreed at its First General Assembly on the need for a Commission on Crystallographic Nomenclature. The present composition of the Commission originated in a resolution by the Fifth General Assembly that its members should be restricted to the Editors of its primary publications. The Commission's size has gradually increased over the years to its present total of 16 ex officio members all of whom are Editors but two, who are chairs of other relevant IUCr bodies.

The Commission's recommendations in its early years took the form of verbal reports presented at triennial IUCr Congresses. The first report of the Commission published in Acta Crystallographica appeared in 1977. Since then, five other reports have been published. Each report generally contains a series of related recommendations; these have ranged from extended definitions of polytypism in a 1984 report to the definition and use of statistical descriptors in 1989 to symbols for symmetry elements and symmetry operations in 1992, among others. Commission recommendations are regarded by the Union as binding on all IUCr publications, and are generally observed by all our sister international organizations as well.

The Commission's character is designed to encourage perceptive recognition of emerging nomenclature problems as they develop in the literature. In the event that divergent naming and notation choices appear that are judged to have the potential for causing serious confusion, the Commission's mechanism of choice for settling the disagreement is to appoint an expert subcommittee charged with studying the matter and presenting appropriate recommendations to the Commission for possible revision and subsequent approval. In recent years, all approved nomenclature recommendations have, in addition, been submitted to the Executive Committee for further revision, if necessary, followed by final acceptance before being published in Acta Crystallographica, Section A. Each subcommittee devotes great effort to its task of analyzing the nomenclature in question and of developing the clearest possible recommendations for future use. No report has been completed in less than two years, most have taken three or four years work, and one report took five years.

Four nomenclature areas are currently under intensive study. The longest-lived of these investigations is that of the Subcommittee on the Nomenclature of N-Dimensional Crystallography, now chaired by T. Janssen of Nijmegen. The Subcommittee is charged with assessing the extent to which the several systems of representational symbolism in this rapidly growing field may have become unacceptably ambiguous and, if the results so warrant, with proposing a set of recommendations for a unified nomenclature and notation system. It is now concentrating on the task of reconciling the major nomenclatures in current use in this literature, leading to a system capable of application as the field continues to evolve.

The Working Group on Statistical Descriptors, chaired by D. Schwarzenbach of Lausanne, was charged with examining the new ISO document Guide to the Expression of Uncertainty in Measurement and with preparing such recommendations as may be appropriate for presentation to the crystallographic community. Formed in March 1993, this Working Group of five people completed eight rounds of revising its report before submitting it to the Commission. The final report presents, inter alia, the major reasons for recommending that the term "estimated standard deviation" be replaced either by "standard uncertainty" or "combined standard uncertainty," depending on the number of uncertainty components evaluated, in statements of the statistical uncertainties of data and the results of measurement.

The charge to the recently established Subcommittee on Atomic Displacement Factor Nomenclature, chaired by K. Trueblood of Los Angeles, is to examine the merits of adopting a uniform approach to reporting atomic displacement factors in structural papers and with making such recommendations as it may find appropriate. The motivation for forming this subcommittee springs from the widespread confusion in the literature regarding the use of the terms U, B, and b and their coordinate systems, particularly in low symmetry crystals. The chairman has outlined the extent of the field that the subcommittee should consider. They propose to consult widely, and welcome the views of those concerned. The subcommittee hopes to complete deliberations and recommendations at an early date.

The Working Group on Phase Transition Nomenclature, with P. Tolédano of Amiens as chairman, is the most recently formed subcommittee. It is charged with studying the multiple systems in current use for naming the sequence of phases that a material may form as a function of temperature and/or pressure and with making whatever recommendations may be appropriate. The literature contains numerous disparate naming systems including I, II, III, etc., a, b, g, etc., and many others, with some using I or a for the highest temperature/pressure phase while others use I or a for the lowest temperature/pressure phase and others that merely use the attribute "low" or "high". IUPAC and IUPAP were each invited to and have appointed a liaison to the Working Group.

The views of members of the crystallographic community on any aspect of crystallographic nomenclature would be warmly welcomed either by the chair of any subcommittee named above or by the undersigned at the Internet address: sca@cauchy.sosc.osshe.edu.