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This article examines the Library of Congress Subject Headings (LCSH), which is the most used subject heading system in the world and an instance of a controlled vocabulary (CV).

The method used to examine the system is based on both authors’ subject knowledge in the field of information science (IS) and the subfield of knowledge organization (KO). Core concepts in this domain were examined (1) by checking if they are present or not in the system; (2) if not, by determining whether LCSH contains alternative terms useful for searching documents about the missing concept, by examining books indexed by the Library of Congress; (3) by identifying the semantic relations between subject headings.

The results demonstrate fundamental problems in the logical consistency of the representation of IS and KO in LCSH.

The implications for CVs in general are discussed.

No previous study has used our method to examine LCSH’s coverage of IS.

This paper attempts to explain the wide dissemination of Library of Congress Subject Headings (LCSH) within digital libraries and presents some of the advantages and disadvantages of using this controlled vocabulary in digital collections. The paper also presents other classifications used in digital collections for subject access and explores ways of improving search functionality in digital collections that employ LCSH.

Unlike traditional libraries that use Library of Congress Classification for organization and retrieval, digital libraries use metadata forms for organization and retrieval. The collections exist in cyberspace of the internet which is known for containing the universe of knowledge. The use of LCSH for information retrieval has been widely criticized for its difficulty of use and its information retrieval effectiveness in online environments. The Library of Congress (LOC) has claimed the headings were not based on comprehensive principles nor ever intended to cover the universe of knowledge. Despite these claims and criticisms, LCSH is the most popular choice for subject access in digital libraries.

The number of digital collections increases every year and LCSH is still the most popular choice of controlled vocabulary for subject access. Of the numerous criticisms, difficulties of use and user unfamiliarity are the greatest disadvantages of using LCSH for subject access. Average users only have a vague notion of what they are looking for when initializing a search. More work is required in automated generation of subject headings and increased usage of LCSH in faceted search retrieval systems. This will provide users with better access to the LCSH used in the back end of information retrieval.

The Greek researchers who developed the Dissertation DSPace system believe this type of module will eventually replace the traditional keyword‐based indexing back ends employed by many information retrieval modules within current digital library systems. The system offers the type of access and interactivity that will acquaint users with how LCSH looks and is used. Faceted search and automated pattern matching using an ontology based on LCSH have the best promise of overcoming the disadvantages that have always plagued the LOC‐controlled vocabulary. These retrieval techniques give LCSH an opportunity to finally achieve the optimal precision and recall it has so far failed to deliver.

The purpose of this paper is to investigate the linking of a folksonomy (user vocabulary) and LCSH (controlled vocabulary) on the basis of word matching, for the potential use of LCSH in bringing order to folksonomies.

A selected sample of a folksonomy from a popular collaborative tagging system, Delicious, was word‐matched with LCSH. LCSH was transformed into a tree structure called an LCSH tree for the matching. A close examination was conducted on the characteristics of folksonomies, the overlap of folksonomies with LCSH, and the distribution of folksonomies over the LCSH tree.

The experimental results showed that the total proportion of tags being matched with LC subject headings constituted approximately two‐thirds of all tags involved, with an additional 10 percent of the remaining tags having potential matches. A number of barriers for the linking as well as two areas in need of improving the matching are identified and described. Three important tag distribution patterns over the LCSH tree were identified and supported: skewedness, multifacet, and Zipfian‐pattern.

The results of the study can be adopted for the development of innovative methods of mapping between folksonomy and LCSH, which directly contributes to effective access and retrieval of tagged web resources and to the integration of multiple information repositories based on the two vocabularies.

The linking of controlled vocabularies can be applicable to enhance information retrieval capability within collaborative tagging systems as well as across various tagging system information depositories and bibliographic databases.

This is among frontier works that examines the potential of linking a folksonomy, extracted from a collaborative tagging system, to an authority‐maintained subject heading system. It provides exploratory data to support further advanced mapping methods for linking the two vocabularies.