The Index to the Dewey Decimal systems, a relative one (i.e., one having cross-references), arranges all topics expressed or implied, with every synonym in alphabetical order. The names Dewey Decimal Classification and Dewey (when referring to the former) are trademarks of the OCLC Online Computer Library Center. This Introduction explains the basic principles and structure of the Dewey Decimal Classification (DDC) system. The Introduction is intended to be used in conjunction with the Glossary and the Manual. The Glossary defines terms used in the Introduction and elsewhere in the Classification.