Objectives: The paper presents a new tendency of using bibliographic  and authority information gathered by the national libraries in Europe. Because of increasing popularity of Linked Open Data there is more and more projects which main purpose is to open and link the bibliographic and authority data from national libraries with other data on the Internet. The main objective of the work is to examine which knowledge organization systems are used in those kind of projects and which of them are the most popular. Also what kind of data formats and ways of publishing data are used.

Methods: The paper is based on the analysis of European national libraries projects' websites and literature about this topic.

Main results and conclusions: Among projects undertaken by the national libraries in Europe there are two main tendencies to bring well organized bibliographic and authority data to the world of Semantic Web. First is to publish national bibliographic and also authority data as open data freely available for everyone on the Internet. The data are structured and shared mostly as data bases or SPARQL endpoint. The most popular data formats are Resource Description and Framework (RDF), JavaScript Object Notation (JSON) and Turtle. Most of the libraries open their data in more than one format (for example British Library shares the British National Bibliography data in RDF, JSON  and Turtle, Polish National Library in XML, JSON, MARCXML and MARC).

Second trend is to link the open data with other data available on the Web. According to OCLC survey conducted in 2014 and 2015 the most frequently used ontologies are the Simple Knowledge Organization System, Friend of a Friend (foaf), DCMI Metadata Terms (dcterms), Dublin Core Metadata Element Set (dce), Schema.org vocabulary (schema) The Bibliographic Ontology (bibo), Local Vocabulary, VOCABS rda, Europeana Data Model vocabulary (edm), ISBD elements (isbd), WGS84 Geo Positioning (geo), BIBFRAME Vocabulary (bf). The data are most commonly associated with such projects as (in order of popularity): VIAF, DBpedia, GeoNames, id.loc.gov. In addition, the British Library connects its data with collections such as Lexvo, country codes, MARC languages codes, and Dewey.info.

Conferring to Tim Berners Lee 5-star deployment scheme for Open Data the highest level of Web data is linked open data. This kind of information probably will lead us to the Semantic Web (where the machines can “understand” the contents of information). National libraries, which are the most predestined to create well organized information, in the digital age should take advantage of being noticed on the Web through high quality of information connected to existing and popular ontologies. Through projects of linking and opening bibliographic and authority data libraries also take part in creating new and improving existing ontologies, schemas and dictionaries.

Keywords: National libraries; Linked Open Data; Semantic Web; Resource Description Framework; Knowledge Organization Systems