Entry Vocabulary and Index Vocabulary 

Terms linked by equivalence relationships are rather different from those in the other two groups (hierarchical and associative relationships). Here we select one preferred term and use only that  in our index, whereas the other two kinds of relationships occur between terms which are both used in the index. In our indexing language we will, therefore, have both: the preferred terms, which are used for indexing, and the non-preferred terms, which are not. The preferred terms on their own form the 'index vocabulary', while the preferred and the non-preferred terms together form the 'entry vocabulary'. The entry vocabulary is very important. There will be many occasions when we decide, for one reason or another the 'not  to use a particular term', but to use the one already in the index vocabulary, instead. The terms in  the entry vocabulary should reflect not only literary warrant but also enquiry warrant. In other words, not only those terms found in the literature, but also those used by readers looking for information. We must be aware of the  terms used by the users of our information retrieval system as well as those used by the authors whose works we are indexing.