International Tables for Crystallography

Specification of a relational dictionary definition language (DDL2)
J. D. Westbrook, H. M. Berman and S. R. Hall. International Tables for Crystallography (2006). Vol. G, ch. 2.6, pp. 61-70  [ doi:10.1107/97809553602060000732 ]


The dictionary definition language version 2 (DDL2) extends the version 1 DDL used by the IUCr for the description of data items common to all crystallographic studies (i.e. core items). The DDL2 extensions were introduced primarily to address two issues arising during the development of a CIF dictionary for the terminology of macromolecular crystallography: the need to accurately describe the hierarchical nature of macromolecular structure and associated structural features, and the desire to encode dictionary definitions in a manner that would permit more detailed software-driven validation. This chapter presents the attribute sets described by the DDL2 language and illustrates their relational character. DDL2 attributes are classified in categories with unique key items; these categories play the roles of tables in a relational database structure. The relationships between categories are illustrated graphically and are described in detail in the chapter. The organization of DDL2 reflects the organization of the data model the attributes of which it describes (i.e. in the mmCIF and related data dictionaries). Consequently, the dictionary of DDL2 attributes is itself constructed in the DDL2 formalism. The use of a self-referential data description scheme allows the consistency and relational integrity of the data model to be independently verified.

Access, prices and ordering

International Tables for Crystallography is available online as a full set of volumes through Wiley InterScience.


If you have already registered and are using a computer listed in your registration details, please email for assistance.

About International Tables for Crystallography

International Tables for Crystallography is the definitive resource and reference work for crystallography. The series consists of eight volumes and comprises articles and tables of data relevant to crystallographic research and to applications of crystallographic methods in all sciences concerned with the structure and properties of materials.