Chapter 2.6 Specification of a relational dictionary definition language (DDL2)

Definition and exchange of crystallographic data

First Online Edition (2006)

Part 2. Concepts and specifications

  1. J. D. Westbrook1,
  2. H. M. Berman1,
  3. S. R. Hall2

Published Online: 1 JAN 2006

DOI: 10.1107/97809553602060000732

International Tables for Crystallography

International Tables for Crystallography

How to Cite

Westbrook, J. D., Berman, H. M. and Hall, S. R. 2006. Specification of a relational dictionary definition language (DDL2). International Tables for Crystallography. G:2:2.6:61–70.

Author Information

  1. 1

    Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers, The State University of New Jersey, Department of Chemistry and Chemical Biology, 610 Taylor Road, Piscataway, NJ 08854‐8087, USA

  2. 2

    School of Biomedical and Chemical Sciences, University of Western Australia, Crawley, Perth, WA 6009, Australia

Publication History

  1. Published Online: 1 JAN 2006



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.


  • DDL2 specification;
  • dictionary definition language;
  • dictionaries;
  • data definitions;
  • categories;
  • methods