A History
By definition, the Universal Decimal Classification (UDC) is an indexing and retrieval language in the form of a classification for the whole of recorded knowledge, in which subjects are symbolized by a code based on Arabic numerals.
[1] The UDC was the brain-child of the two Belgians, Paul Otlet and Henry LaFontaine, who began working on their system in 1889, 15 years after Melvil Dewey established the DDC.
[2] Otlet and LaFontaine built their system on the foundation of the DDC with Melvil Dewey’s express permission. While Dewey conceived his scheme to be applied to the arrangement of books on shelves, Otlet and LaFontaine, whose fields were Sociology, Law, Statistics, Political Economy, as well as Philology and Literature, were ultimately more interested in journal articles, news items, other related documents, and how to access them. Thus, they required a more detailed system. Fortunately,Dewey agreed to allow them to apply his system to the International Index they had conceived, and by 1895 they had amassed and classified 400,000 cards for their Universal Index. Their system caught on after presenting it to a conference held the same year. Otlet and LaFontaine were required to augment Dewey’s system with numerous devices that they later described as synthetic.
In 1920, a Dutch chemical engineer by the name of Donker Duyvis became the secretary of the editorial panel for the second (French) edition of the UDC, ushering in what was known in the history of the UDC as the authoritative or dictatorial period. Duyvis believed that classification was the necessary liaison between “Efficiency, “”Standardization” and “Information.” With this in mind he initiated the preparation of a new edition of the UDC in 1922/23. Along with Otlet and La Fontaine, he formed the editorial board for the new UDC edition, preparing the second French edition between 1926 and 1931. This edition formed basis for all ensuing versions.
Under Duyvis guidance, the seat of all UDC activities was moved from Brussels to The Hague, and in 1931 the Institut International de Biblographie (IIB), the parent of the UDC, officially became the Institut International de Documentation (IID). Then in 1938, “Documentation” was replaced by “Federation.” The name “Federation International de Documentation” remains to this day.
Since the UDC emerged from a practical need there appears to be no actual principle underlying it, except that it suited the needs for which it was created. Nevertheless, it is in the strictest sense a classification contingent on the analysis of idea contents. It is a comprehensive classification covering most areas of knowledge. Moreover, the UDC is considered a documentary classification that originated from an enumerative system to a faceted one. And while it was created for bibliographic purposes it has proven suitable for libraries. The UDC covers every field of knowledge as an integrated system of correlated subjects, and like the DDC it is constructed on the general to specific principle of applying decimal divisions. The documents that embody any form of knowledge: be they literature, i.e. written documents, as well as other media such as films, video, sound recordings, illustrations, maps, and even such objects as museum pieces. All are suitable for classification according to the UDC. It classes a phenomenon according to the concept or discipline in which it is considered.
[3] According to Robinson, the UDC code may be applied in two ways: (1) It may be transcribed directly on to the documents, and be used to determine their physical arrangement: for instance, books on shelves, papers in a file, or paragraphs in a book; or (2) It may be included in the references to the documents: for instance, in entries in catalogues, or bibliographies. In many systems, the two methods can be usefully combined. In this way, all information, or references to information about a particular subject are brought together; they can then be located and retrieved with the minimum of searching.

[4] The Editions
Like the DDC, the UDC has undergone expansion, revision and modification several times in its 80 year lifetime. The first edition of the UDC appeared in French between 1904 and 1907, and consisted of approximately 33,000 subdivisions. This number increased to about 140,000 in its third edition which was published in German between 1934 and 1951, and comprised seven volumes of tables and three of indices. A single-volume abridged English edition, the UDC (A) was published in 1961 and contained some 14,000 subdivisions. Currently, there is a two-volume Medium Edition English Text, the UDC(M), which includes over 40,000 subdivisions, and is more systematic than the UDC (A).
[5] The UDC has been translated into over thirty languages and has been widely used all over the world. Printed editions exist in a range of sizes from pocket editions (e.g. French, English), the standard edition (e.g. Spanish, French, English), or expanded versions (e.g. Russian). Electronic versions are similarly available in various languages and formats.
[6] The Structure of the Universal Decimal System
The custodians of the UDC claim that its most innovative and influential feature is its ability to express not just simple subjects but relations between subjects. This facility is added to a hierarchical structure: knowledge is then divided into ten classes. Each class is subdivided into its logical parts, with each subdivision further subdivided. The more detailed the subdivision, the longer the number that represents it. Like the DDC, this is made possible by decimal notation.
In the Universal Decimal Classification all recorded knowledge is treated as a coherent system, which is built of related parts. This is in contrast to a specialized classification in which related subjects are treated as subsidiary even though they may be of major importance in their own right. As a result, specialists may be led to related information of which they would otherwise have been unaware. Notation and arrangement
Another feature the UDC shares with the DDC is that the symbols chosen for its notation are non-language-dependent, and universally recognizable - the Arabic numerals, which are supplemented by a few other signs derived from mathematics and ordinary punctuation. While they may not be easy to read, they are easy to transcribe using ordinary office machinery such as typewriters and computer keyboards.
The arrangement is based on the decimal system: every number is thought of as a decimal fraction with the initial point omitted. This determines its filing order. To facilitate reading, however, it is usually punctuated after every third digit (see examples below). One advantage of this system is that it is infinitely extensible; when new subdivisions are introduced, they will not disturb the existing allocation of numbers. The Tables
[7] The UDC employs two kinds of tables:
1. The Main Tables, which are also called the 'schedules'.
These contain the outline of the various disciplines of knowledge, and are arranged in 10 classes and hierarchically divided and numbered from 0 to 9. A unique feature of the UDC is its use of zeros. One and two zeros can be used as connecting symbols, facet indicators, for numbers of special auxiliaries.
[8] Apparently, even more important is the UDC’s simultaneous use of zeros as rich digits to denote specific ideas. This application of zeros came about as a result of the inherent limitations of the UDC’s notational base, and marked a departure from the DDC’s minimum three-digit classification.
2. The Auxiliary Tables. The large number of auxiliary tables, which add facetedness to the system are considered perhaps one of the UDC’s most outstanding features. These include certain auxiliary signs, connecting symbols, such as the plus (+), the stroke (/), the apostrophe (‘), and the colon(:), which are used to link two (or more) numbers, thus expressing the relations of various kinds between two (or more) subjects. The system also makes use of square brackets ([ ]) to denote subordinate concepts, as well as the asterisk (*), which was applied to non-decimal numerical subdivisions as a prefix. The enumerative tables denote recurrent characteristics, and are applicable over a range of subjects; the auxiliary is simply added at the end of the number for the subject. The most general of them are called common auxiliaries. These are applied throughout the main tables, and represent such notions such as place, language of the text, and physical form of the document, which may occur in almost any subject. In addition, there are more restricted series that are called special auxiliaries. These express aspects that are recurrent, but in a more limited way in terms of subject range. They can be found only in particular sections of the main tables. A significant addition to the UDC is the possibility to precisely construct class numbers with these added auxiliaries. The UDC and Technology
The digital nature of the UDC commends it to computer technology. Browsing is facilitated in an on screen format. The UDC's distinctive symbols make it possible to perform searches for any part of a compound number or for specified combinations of symbols, thus providing highly accurate subject retrieval. Indeed, according to the official website, UDC's combination of numerical codes and natural-language descriptions makes it amenable to numerical and alphabetic sorting, in maintaining tools such as catalogues, authority files and indexes. A core version of UDC - the Master Reference File, with more than 65,000 subdivisions is now available in database format. The international foundation of the UDC and its supporting consortium, UDCC, has created an international (and eventually multilingual) database, and the UDC is also available on in electronic form on the web in addition to CD-ROM format.