Editing Universal Decimal Classification (section)

== UDC structure ==

=== Notation ===
A notation is a code commonly used in classification schemes to represent a class, i.e. a subject and its position in the hierarchy, to enable mechanical sorting and filing of subjects. UDC uses [[Arabic numerals]] arranged decimally. Every number is thought of as a decimal fraction with the initial decimal point omitted, which determines the filing order. An advantage of decimal notational systems is that they are infinitely extensible, and when new subdivisions are introduced, they need not disturb the existing allocation of numbers. For ease of reading, a UDC notation is usually punctuated after every third digit:
{| 
|- 
| style="width:18%; font-weight:bold;" | Notation || style="font-weight:bold;" | Caption (Class description)
|-
| 539.120 ||Theoretical problems of elementary particles physics. Theories and models of fundamental interactions
|-
| 539.120.2 || Symmetries of quantum physics
|-
|539.120.22 ||  Conservation laws
|-
| 539.120.222 ||   Translations. Rotations
|-
| 539.120.224 ||   Reflection in time and space
|-
|539.120.226 ||   Space-time symmetries
|-
| 539.120.23 ||  Internal symmetries
|-
| 539.120.3 || Currents
|-
| 539.120.4 || Unified field theories
|-
|539.120.5 || Strings
|}

In UDC the notation has two features that make the scheme easier to browse and work with: 
* '''hierarchically expressive''' – the longer the notation, the more specific the class: removing the final digit automatically produces a broader class code.
* '''syntactically expressive''' – when UDC codes are combined, the sequence of digits is interrupted by a precise type of punctuation sign which indicates that the expression is a combination of classes rather than a simple class e.g. the colon in 34:32 indicates that there are two distinct notational elements: 34 Law. Jurisprudence and 32 Politics; the closing and opening parentheses and double quotes in the following code 913(574.22)"19"(084.3) indicate four separate notational elements: 913 Regional geography, (574.22) North Kazakhstan (Soltüstik Qazaqstan); "19" 20th century and (084.3) Maps (document form)

=== Basic features and syntax ===
UDC is an analytico-synthetic and [[faceted classification]]. It allows an unlimited combination of attributes of a subject and relationships between subjects to be expressed. UDC codes from different tables can be combined to present various aspects of document content and form, e.g. 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.<ref name="UDC Subject Coverage">[http://www.udcc.org/index.php/site/page?view=subject_coverage UDC Subject Coverage], UDC Consortium website</ref> 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:<ref name="UDC Structure">[http://www.udcc.org/index.php/site/page?view=about_structure UDC Structure and Tables], UDC Consortium website</ref>

*'''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 [[Outline of philosophy|Philosophy]]. [[Outline of psychology|Psychology]]
*2 [[Outline of religion|Religion]]. [[Outline of theology|Theology]]
*3 [[Outline of social science|Social Sciences]]
*4 ''vacant''
*5 [[Outline of mathematics|Mathematics]]. [[Outline of natural science|Natural Sciences]]
*6 [[Outline of applied science|Applied Sciences]]. [[Outline of medicine|Medicine]], [[Outline of technology|Technology]]
*7 [[The arts|The Arts]]. [[Outline of entertainment|Entertainment]]. [[Outline of sports|Sport]]
*8 [[Outline of linguistics|Linguistics]]. [[Outline of literature|Literature]]
*9 [[Outline of geography|Geography]]. [[Outline of history|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 the rapidly expanding fields of knowledge; primarily natural sciences and technology.

==== Common auxiliary tables ====
''Common auxiliaries'' are aspect-free concepts that can be used in combination with any other UDC code from the main classes or with other common auxiliaries. They have unique notational representations that make them stand out in complex expressions. Common auxiliary numbers always begin with a certain symbol known as a facet indicator, e.g. <nowiki>=</nowiki> (equal sign) always introduces concepts representing the language of a document; (0...) numbers enclosed in parentheses starting with zero always represent a concept designating document form. Thus (075) Textbook and =111 English can be combined to express, e.g.(075)=111 Textbooks in English, and when combined with numbers from the main UDC tables they can be used as follows: 2(075)=111 Religion textbooks in English, 51(075)=111 Mathematics textbooks in English etc.

*=...	Common auxiliaries of language. Table 1c
*(0...)	Common auxiliaries of form. Table 1d
*(1/9)	Common auxiliaries of place. Table 1e
*(=...)	Common auxiliaries of human ancestry, ethnic grouping and nationality. Table 1f
*"..."	Common auxiliaries of time. Table 1g helps to make minute division of time e.g.:  "1993-1996"
*-0...	Common auxiliaries of general characteristics: Properties, Materials, Relations/Processes and Persons. Table 1k.
*-02	Common auxiliaries of properties. Table 1k
*-03	Common auxiliaries of materials. Table 1k
*-04	Common auxiliaries of relations, processes and operations. Table 1k
*-05	Common auxiliaries of persons and personal characteristics. Table 1k this table is repeated

==== Connecting signs ====
In order to preserve the precise meaning and enable accurate parsing of complex UDC expressions, a number of connecting symbols are made available to relate and extend UDC numbers. These are:
{| class="wikitable"
!Symbol !! Symbol name !! Meaning !! Example
|-
|<nowiki>+</nowiki> || [[Plus and minus signs|plus]] || coordination, addition || e.g. 59+636 [[zoology]] and [[animal breeding]]
|-
|<nowiki>/</nowiki> || [[Slash (punctuation)|stroke]] || consecutive extension || e.g. 592/599 Systematic zoology (everything from 592 to 599 inclusive)
|-
|<nowiki>:</nowiki> || [[Colon (punctuation)|colon]] || relation || e.g. 17:7  Relation of [[ethics]] to [[art]]
|-
|<nowiki>[ ]</nowiki> || square [[bracket]]s || subgrouping || e.g. 311:[622+669](485) [[statistics]] of [[mining]] and [[metallurgy]] in [[Sweden]] (the auxiliary qualifiers 622+669 considered as a unit)
|-
|<nowiki>*</nowiki> || asterisk || Introduces non-UDC notation  || e.g. 523.4*433 Planetology, minor planet Eros ([[International Astronomical Union|IAU]] authorized number after the asterisk)
|-
|<nowiki>A/Z</nowiki> || alphabetical extension || Direct alphabetical specification  || e.g. 821.133.1MOL French literature, works of Molière
|}