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Library classification forms part of the field of library and information science. It is a form of bibliographic classification (library classifications are used in library catalogs, while "bibliographic classification" also covers classification used in other kinds of bibliographic databases). It goes hand in hand with library (descriptive) cataloging under the rubric of cataloging and classification, sometimes grouped together as technical services. The library professional who engages in the process of cataloging and classifying library materials is called a cataloger or catalog librarian. Library classification systems are one of the two tools used to facilitate subject access. The other consists of alphabetical indexing languages such as Thesauri and Subject Headings systems.
A library classification is a system of coding and organizing library materials (books, serials, audiovisual materials, computer files, maps, manuscripts, realia) according to their subject and allocating a call number to that information resource. Similar to classification systems used in biology, bibliographic classification systems group entities that are similar together typically arranged in a hierarchical tree structure. A different kind of classification, called a 'faceted' system, is also widely used. This synthesises a class mark from various aspects of the subject.
Classification of a piece of work consists of two steps. Firstly the 'aboutness' of the material is ascertained. Next, a call number,(essentially a book's address), based on the classification system will be assigned to the work using the notation of the system.
It is important to note that unlike subject heading or Thesauri where multiple terms can be assigned to the same work, in classification systems, each work can only be placed in one class. This is due to shelving purposes: A book can have only one physical place. However in classified catalogs one may have main entries as well as added entries. Most classification systems like DDC and Library of Congress classification, also add a cutter number to each work which adds a code for the author of the work.
Classification systems in libraries generally play two roles. Firstly they facilitate subject access by allowing the user to find out what works or documents the library has on a certain subject. Secondly, they provide a known location for the information source to be located (e.g where it is shelved).
Until the 19th century, most libraries had closed stacks, so the library classification only served to organize the subject catalog. In the 20th century, libraries opened their stacks to the public and started to shelve the library material itself according to some library classification to simplify subject browsing.
Some classification systems are more suitable for aiding subject access, rather than for shelf location. For example, UDC which uses a complicated notation including plus, colons are more difficult to use for the purpose of shelf arrangement but are more expressive compared to DDC in terms of showing relationships between subjects. Similarly faceted classification schemes are more difficult to use for shelf arrangement, unless the user has knowledge of the citation order.
Depending on the size of the library collection, some libraries might use classification systems solely for one purpose or the other. In extreme cases a public library with a small collection might just use a classification system for location of resources but might not use a complicated subject classification system. Instead all resources might just be put into a couple of wide classes (Travel, Crime, Magazines etc). This is known as a "mark and park" classification method, more formally called reader interest classification 
There are many standard system of library classification in use, and many more have been proposed over the years. However in general, Classification systems can be divided into three types depending on how they are used.
- Universal schemes covering all subjects. Examples include Dewey Decimal Classification, Universal Decimal Classification and Library of Congress Classification
- Specific classification schemes. Examples include Iconclass, British classification of Music, and Dickinson classification.
- National schemes specially created for certain countries. An example is the Swedish library classification system, SAB (Sveriges Allmänna Biblioteksförening).
In terms of functionality, classification systems are often described as
- enumerative: produce an alphabetical list of subject headings, assign numbers to each heading in alphabetical order
- hierarchical: divides subjects hierarchically, from most general to most specific
- faceted or analytico-synthetic: divides subjects into mutually exclusive orthogonal facets
There are few completely enumerative systems or faceted systems, most systems are a blend but favouring one type or the other. The most common classification systems, LCC and DDC, are essentially enumerative, though with some hierarchical and faceted elements (more so for DDC), especially at the broadest and most general level. The first true faceted system was the Colon classification of S. R. Ranganathan.
Universal classification systems used in English-speaking world
- Bliss bibliographic classification (BC)
- Dewey Decimal Classification (DDC)
- Library of Congress Classification (LCC)
(The above systems are the most common in the English-speaking world.)
- Harvard-Yenching Classification - An English classification system for Chinese language materials.
Universal classification systems in other languages
- A System of Book Classification for Chinese Libraries (Liu's Classification)
- Nippon Decimal Classification (NDC)
- Chinese Library Classification (CLC)
- Korean Decimal Classification (KDC)
Universal classification systems that rely on synthesis (faceted systems)
- Bliss bibliographic classification
- Colon classification
- Cutter Expansive Classification
- Universal Decimal Classification
- Brinkler classification
Newer classification systems tend to use the principle of synthesis (combining codes from different lists to represent the different attributes of a work) heavily, which is comparatively lacking in LC or DDC.
Comparing Classification Systems
As a result of differences in Notation, history, use of enumeration, hierarchy, facets, classification systems can differ in the following ways
- Type of Notation - Notation can be pure (consisting of only numerals for example) or mixed (consisting of letters, numerals, and other symbols).
- Expressiveness - This is the degree in which the notation can express relationship between concepts or structure.
- Whether they support mnemonics - For example the number 44 in DDC notation usually means it concerns some aspect of France. For example 598.0944 concerns "Birds in France". the 09 signifies country code, and 44 represents France.
- Hospitality - The degree in which the system is able to accommodates new subjects
- Brevity - Length of the notation to express the same concept
- Speed of updates and degree of support - The best classification systems are constantly being reviewed and improved.
- Chan, Lois Mai. Cataloging and Classification: An Introduction. New York: McGraw-Hill, 1994.
- An annotated bibliography on Library Classification and Subject Approaches to Information - by Salman Haider, Librarian (Cataloging), US Library of Congress, New Delhi Overseas Office
- A Preview of the New Cataloging Code
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