Types of Thesauri
Based on the nature of terminology control, there are mainly two types of thesauri:
a) controlled thesauri which allow only one term (preferred term) to denote a concept for the purpose of indexing and searching; and
b) free language thesauri, which allow use of all terms to denote a concept to be used for indexing and searching.
The controlled thesauri can be maintained manually but free language thesauri require machine maintenance and retrieval.