The capacity of a DDL to precisely define data items depends implicitly on the scope of the available attributes. It is quite possible, therefore, that a completely new data property cannot be specified using an existing DDL. In a field where data types evolve rapidly, the currently used dictionary language may be inadequate for the precise specification of an item. It is inevitable that future data items will exceed the capacity of existing dictionary attributes and methods to describe new data properties, and the dictionaries must evolve much in the same way that spoken languages continually adapt to changing modes of expression.

The first DDL used in crystallography (in 1990) was developed to compile a dictionary of `core structural' crystallographic data items (Hall et al., 1991). These data items were intended for use when submitting a manuscript to the journal Acta Crystallographica (and still are). The `core' DDL is known commonly as DDL1 (Hall & Cook, 1995). Several years later, the definition of macromolecular crystallographic data items needed hierarchical descriptors for the different levels of structural entities, and an `mmCIF' DDL, known as DDL2 (Westbrook & Hall, 1995), was developed. DDL2 was used to build the mmCIF dictionary (Bourne et al., 1997). DDL1 is described in this chapter and DDL2 is described in Chapter 2.6 . The DDL1 attributes have been used to construct the crystallographic core (fundamental structural), pd (powder diffraction), ms (modulated and composite structures) and rho (electron density) dictionaries. These dictionaries are discussed in Chapters 3.2–3.5 and presented in full in Chapters 4.1–4.4, respectively, of this volume.

It is helpful to give a brief comparison of the definition capabilities of DDL1 and DDL2. The differences may be best illustrated by simply comparing their application to the definition of the same data item. The two separate definitions of _atom_site_Cartn_x are shown in Fig. 2.5.4.1.

Figure 2.5.4.1 | top | pdf | Comparison of DDL versions: (a) DDL1, (b) DDL2.

Although both DDL versions conform to the syntax of the STAR File (DDL1 conforms to the CIF syntax but DDL2 uses STAR File save frames not permitted in CIF data files) and are composed of similar attributes, there are clear differences. These are summarized in Table 2.5.4.1.

The similarity in the two DDL versions is such that software exists for parsing and validating CIFs against the DDL1 or DDL2 dictionaries interchangeably (e.g. see the CIF toolbox software CIFtbx described in Chapter 5.4 ).