1. International Standards Organization 2006 ISO 11179-2 Information technology - Metadata registries (MDR). Part 2. Classification http://metadata-standards.org/11179/
2. International Standards Organization 2011 ISO 25964 Thesaurus schemas www.niso.org/schemas/iso25964/
3. American National Standards Institute 2010 ANSI/NISO Z39.19 (R2010) Guidelines for the construction, format, and management of monolingual controlled vocabularies www.niso.org/apps/group_public/project/details.php?project_id=46
4. The Joy of Sets