94(410)"19"(075) History (main subject) of United Kingdom (place) in 20th century (time), a textbook (document form). Or: 37:2 Relationship between Education and Religion. Complex UDC expressions can be accurately parsed into constituent elements. UDC is also a disciplinary classification covering the entire universe of knowledge. This type of classification can also be described as aspect or perspective, which means that concepts are subsumed and placed under the field in which they are studied. Thus, the same concept can appear in different fields of knowledge. This particular feature is usually implemented in UDC by re-using the same concept in various combinations with the main subject, e.g. a code for language in common auxiliaries of language is used to derive numbers for ethnic grouping, individual languages in linguistics and individual literatures. Or, a code from the auxiliaries of place, e.g. (410) United Kingdom, uniquely representing the concept of United Kingdom can be used to express 911(410) Regional geography of United Kingdom and 94(410) History of United Kingdom. Organization of classes Concepts are organized in two kinds of tables in UDC: Common auxiliary tables (including certain auxiliary signs). These tables contain facets of concepts representing general recurrent characteristics, applicable over a range of subjects throughout the main tables, including notions such as place, language of the text and physical form of the document, which may occur in almost any subject. UDC numbers from these tables, called common auxiliaries are simply added at the end of the number for the subject taken from the main tables. There are over 15,000 common auxiliaries in UDC. The main tables or main schedules containing the various disciplines and branches of knowledge are arranged in 9 main classes, numbered from 0 to 9 (with class 4 being vacant). At the beginning of each class there are also series of special auxiliaries, which express aspects that are recurrent within this specific class. Main tables in UDC contain more than 60,000 subdivisions. Main classes 0 Science and Knowledge. Organization. Computer Science. Information Science. Documentation. Librarianship. Institutions. Publications 1 Philosophy. Psychology 2 Religion. Theology 3 Social Sciences 4 vacant 5 Mathematics. Natural Sciences 6 Applied Sciences. Medicine, Technology 7 The Arts. Entertainment. Sport 8 Linguistics. Literature 9 Geography. History The vacant class 4 is the result of a planned schedule expansion. This class was freed by moving linguistics into class 8 in the 1960s to make space for future developments in