This article begins with a brief description of the indexing language as a factor that affects the efficiency of indexing. After reviewing different indexing languages which are natural indexing language, free indexing language, and controlled indexing language, the article describes the structure and function of thesauruses. Next it provides a short history of thesauruses, inside and outside Iran and then explains the form of thesauruses (the method they use for presenting terms and the semantic relations). The rules of developing thesauruses are studied from two aspects: vocabulary control and determining basic thesaurus relations. Then their structure is studied in terms of the solutions they provide for enhancing precision and recall in information retrieval. Finally, the differences between thesauruses, subject headings and classification schemes are explored.
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