  The UDC (Universal Decimal Classification) is not only a classification
language with a long history; it also presents a complex cognitive system
worthy of the attention of complexity theory. The elements of the UDC: classes,
auxiliaries, and operations are combined into symbolic strings, which in
essence represent a complex networks of concepts. This network forms a backbone
of ordering of knowledge and at the same time allows expression of different
perspectives on various products of human knowledge production. In this paper
we look at UDC strings derived from the holdings of libraries. In particular we
analyse the subject headings of holdings of the university library in Leuven,
and an extraction of UDC numbers from the OCLC WorldCat. Comparing those sets
with the Master Reference File, we look into the length of strings, the
occurrence of different auxiliary signs, and the resulting connections between
UDC classes. We apply methods and representations from complexity theory.
Mapping out basic statistics on UDC classes as used in libraries from a point
of view of complexity theory bears different benefits. Deploying its structure
could serve as an overview and basic information for users among the nature and
focus of specific collections. A closer view into combined UDC numbers reveals
the complex nature of the UDC as an example for a knowledge ordering system,
which deserves future exploration from a complexity theoretical perspective.
