Introduction

The Relative Index of the DDC has been a major part of each edition since the first, in 1876. It is, of course, primarily an adjunct or aid in the use of the schedules, which form the core of DDC. Nevertheless, it has always been a feature that transcends the usefulness of a typical index at the back of a book. The Relative Index is considered by many to be Melvil Dewey's paramount and enduring contribution to library classification and indexing.

Need for and importance of the Relative Index

All terms found in the schedules and the various auxiliary tables have been arranged logically in whole-part and genus-species relations – in other words, in a systematic order that proceeds from the general to the specific. Such arrangements are designed to mirror the structure of knowledge and its evolution, depicting the position of a subject in the universe of knowledge. Locating a subject in the schedules often requires at least a preliminary knowledge of a subject's position in the universe of knowledge. It is a tall order for a single user to have even a preliminary knowledge of all the subjects in the universe of knowledge. Therefore, at one time or another, virtually every DDC user will turn to the Relative Index, the alphabetical key to the schedules and tables. There are several reasons why.

Illogical placements of some subjects in DDC

Despite intentions, in the DDC itself the position of some subjects is neither logical nor in accord with current scholarly consensus. Some misplacements in the original plan of 1876 remain to this day. Its basic structure still draws from a 19th-century worldview of knowledge. In later developments of the system, some new subjects had to be placed in illogical locations as there were no vacant numbers available at their rightful places.

Limited knowledge of the classifier

Due to scholarly advancements and the rapid dissemination of information, the universe of knowledge is much more complex than in Melvil Dewey's day. It is hardly unusual for a classifier to be at first unfamiliar with the subject of a work being classified. At other times, the classifier may understand the subject but may not be aware of its location in the overall system. Take, for example, the subject of birthday cards: one may not even be able to guess where its primary class might fall.