We have print thesauri for many of our indexes and databases. Many electronic indexes have online versions of their thesauri that you can use to construct your search. For help with thesauri, please see a librarian. Free Text Controlled vocabulary is a big help in retrieving relevant records in your search, but it doesn't solve all the problems with language. It takes time to create a controlled vocabulary, and new terms are always coming into use, so the controlled vocabulary for a database may always be a little out of date.
Some terms go in and out of use very quickly and are never added to the controlled vocabulary.
Sometimes no terms in the controlled vocabulary are a good match for the concepts you are searching for. To help solve these problems, database producers include searchable fields for free text terms in database records. Concepts are expressed in free text fields without reference to the database thesaurus. In some fulltext databases, the entire text of the article is a free text field. Authors of the articles, rather than indexers, are usually the ones who decide on the terms in free text fields.