Structure

Like the DDC, the UDC is based on the decimal system: every number is thought of as a decimal fraction with the initial point omitted, and this determines the filing order, but, for ease of reading, it is usually punctuated after every third digit.

An advantage of this system is that it is infinitely extensible, and when new subdivisions are introduced, they need not disturb the existing allocation of numbers.

In UDC, all recorded knowledge is treated as a coherent system, built of related parts, in contrast to a specialized classification, in which related subjects are treated as subsidiary even though in their own right they may be of major importance. Thus users may often be led to related information of which they would otherwise have not found.

Like most general schemes, UDC is an aspect classification. Phenomena are subordinated to the aspect from which they are considered. This means that a phenomenon may occur in more than one class, for example, eggs in ornithology, cooking, animal husbandry, etc.

The UDC is a hierarchical classification, which means that each subdivision may be further subdivided into its logical components. This is done by the application, successively, of principles of division.

The UDC is a synthetic classification, which means that the enumerated classes are the building blocks whereby compound and complex classes may be denoted by the various synthetic notational devices (such as -,=,+,/). Thus the symbols used in the notation are non-language-dependent, and universally recognizable.

The UDC can also act as a switching language between different information systems. A search can be performed using numbers, or the retrieval interface can be enriched with alphabetical subject indexes in many languages or with multilingual thesauri.

The UDC is a faceted classification. However, unlike a truly faceted classification scheme such as the Colon classification, the UDC does not list all of the facets in only one location. Instead, the UDC adopted a more pragmatic approach to this type of classification in order to increase the user-friendliness in number building. In the UDC, faceted components are duplicated in different classes.