The type of index most writers work with is the "back-of-the-book" variety. A global index combines the back-of-the-book indexes from all the books in a set. A global index is a valuable information retrieval device for a reader who is not intimately familiar with all the books in a set. A global index provides a single place where readers can find the information they seek without having to look through several individual indexes.
In a global index, merely referring a reader to page numbers is insufficient. Because a global index combines the indexes from several books in a set, a reader also needs to know the book in the set to which the reference applies. For this reason, a global index requires a special page numbering style. One method is to use a four-letter abbreviation for an entry's book title, with a running footer that provides a legend on each index page for the abbreviations. If running such a footer is not possible or desirable, put a legend for abbreviations at the beginning of the index.
Because of its size and complexity, a global index is by far the most difficult type of index to create properly. It is not sufficient just to combine the indexes from several books and assume that, if the previous back-of-the-book indexes were correct, a global index will also be correct.
When combining indexes to make a global index, the indexer must edit the global index for most of the indexing mistakes described under "Editing an Index." Specifically, the indexer must check for:
Many of these mistakes may creep into a global index even though they may not have been in the original indexes. These mistakes happen when common terms in different indexes are combined, slightly different terminology is used in different books, and different indexing choices have been applied in each book.
There are two ways to fix the problems that result from creating a global index: