Print indexes use controlled vocabulary or subject headings to identify concepts and organize the items indexed. Sometimes these subject headings are published as a formal thesaurus for the index. Cross references are used in both the index and its thesaurus to direct the user to the authorized term for a particular concept.

The subject headings used by one index will not necessarily be the same as the ones used by another index. If you are having trouble finding the right search terms for your topic, an index's thesaurus, if it is available, is a good place to look. It will tell you what terms that index uses for your topic.

NOTE: Language and the meanings of words change over time. New words are added and old words get new definitions. A term that was used to describe a topic 25, 50, or 100 years ago may not be used today. You need to try several different terms and follow the cross references in each volume of the index until you are sure that you have found the most correct subject terms for your topic.
Pay attention: you will find that a set of subject terms which gave you good results during one span of time does not give you good results in the next span of time. When this happens, you need to identify the new subject headings for your topic. Only by changing your search terms with the changes in the language will you find most of the items relevant to your topic.