Indexes often include more than one level of keywords. Multiple levels enable you to logically arrange keywords based on their relationships with other keywords, as shown here:
The top-level keywords (parent keywords) are more general and the subkeywords (subordinate keywords) are more specific. For example, the keyword "Caribbean" is a general classification. The subkeyword "Hotels" divides this keyword into a specific subclass. Similarly, both "Sandy Lane" and "Ocean Club" are subkeywords of "Hotels."
Use a subkeyword if it starts with the same word(s) which denotes a close relationship. For example, "Visiting Aruba," "Visiting Barbados," and "Visiting Hawaii," can be subkeywords grouped under a parent keyword: Visiting: Aruba, Barbados, Hawaii.
Use a subkeyword if it is ambiguous unless used with the context of a parent keyword. For example, "Photographs of" cannot stand alone without a parent keyword such as "Tropical Fish" to provide context.
Use no more that two levels of subkeywords. Try to limit it to one level if possible. Multilevel indexes arrange keywords and subkeywords in an indented format, making it necessary to scroll horizontally in the Index pane if the subentries are excessively lengthy.
Keywords and subkeywords need to be grammatically arranged. Subkeywords can include articles, prepositions and conjunctions for clarity, although it is a good idea to limit them.
Do not use only one subkeyword. There must be two or more.
If a parent keyword is linked to more than two topics, it needs subkeywords.
Do not over-analyze a keyword by adding unnecessary subkeywords that all point to the same topic.