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Since 1998, the NL collections have been indexed in a German indexing language according to the RSWK (Rules for the subject catalogue) indexing standards. Until April 2012 the controlled vocabulary of the SWD (Subject headings authority file) was used to describe the content of a document. Individual SWD subject headings were combined to make subject heading strings (e.g.: Schweiz - Familienzulage - Landwirtschaftlicher Betrieb - Recht - Quelle). The strings were permuted and every subject heading was listed in the alphabetical subject index.
In April 2012 the previously separate authority files GKD (Corporate bodies authority file), PND (Name authority file), SWD (Subject headings authority file) as well as EST (Uniform title file of the Deutsches Musikarchiv) were merged into the new universal authority file GND ("Gemeinsame Normdatei"). Since then descriptors of the GND have been used to index the documents of the NL.
Pre-coordination (subject heading strings) was given up for technical reasons, in order to allow an improved control of the vocabulary.
Before 1998, subject indexing was carried out according to the Universal Decimal Classification (UDC).
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