Tables for
Volume G
Definition and exchange of crystallographic data
Edited by S. R. Hall and B. McMahon

International Tables for Crystallography (2006). Vol. G, ch. 3.1, p. 77

Section Category descriptions

B. McMahona*

aInternational Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England
Correspondence e-mail: Category descriptions

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As discussed above, categories in DDL1 are intended as `natural groupings' of data items. To document the purpose of a category within a dictionary, `pseudo' data names are used. All pseudo data names are assigned a _category attribute of category_overview and have an associated _type value of `null'. They are also named by convention as _category_name_[dictionary code], for example _pd_data_[pd] for the description of the PD_DATA category in the powder dictionary (indicated by the code `pd' in square brackets). For the core dictionary, dictionarycode is not given, resulting in names like _exptl_[] to describe the EXPTL category.

Example[link] is a slightly edited extract from the core dictionary showing how a data block for a category description is composed, including the presence of an example.

Example A category description in a DDL1 dictionary.

[Scheme scheme4]

Note that the dictionarycode extension allows a dictionary to include comments on items that it defines in a category already established in the core dictionary. For example, the modulated structures dictionary includes the category overview item _audit_link_[ms]. This describes the convention adopted to express the relationship between data blocks in a modulated structures data file using the _audit_link_ data names already defined in the core dictionary.

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