Hierarchy
Hierarchy in the DDC is expressed through structure and notation. Structural hierarchy means that all topics (aside from the ten main classes) are part of all the broader topics above them. Any note regarding the nature of a class holds true for all the subordinate classes, including logically subordinate topics classed at coordinate numbers.
Notational hierarchy is expressed by length of notation. Numbers at any given level are usually subordinate to a class whose notation is one digit shorter; coordinate with a class whose notation has the same number of significant digits; and superordinate to a class with numbers one or more digits longer. The underlined digits in the following example demonstrate this notational hierarchy:
600 Technology
630 Agriculture and related technologies
636 Animal husbandry
636.7 Dogs
636.8 Cats
“Dogs” and “Cats” are more specific than (i.e., are subordinate to) “Animal husbandry”; they are equally specific as (i.e., are coordinate with) each other; and “Animal husbandry” is less specific than (i.e., is superordinate to) “Dogs” and “Cats.” Sometimes, other devices must be used to express the hierarchy when it is not possible or desirable to do so through the notation. Special headings, notes, and entries indicate relationships among topics that violate notational hierarchy.
Arrangement of the DDC
The print version of the latest full edition of the DDC, Edition 22, is composed of the following major parts in four volumes:
Volume 1
(A) New Features in Edition 22: A brief explanation of the special features and changes in DDC 22
(B) Introduction: A description of the DDC and how to use it
(C) Glossary: Short definitions of terms used in the DDC
(D) Index to the Introduction and Glossary
(E) Manual: A guide to the use of the DDC that is made up primarily of extended discussions of problem areas in the application of the DDC. Information in the Manual is arranged by the numbers in the tables and schedules
(F) Tables: Six numbered tables of notation that can be added to class numbers to provide greater specificity
(G) Lists that compare Editions 21 and 22: Relocations and Discontinuations;
Reused Numbers
Volume 2
(H) DDC Summaries: The top three levels of the DDC
( I) Schedules: The organization of knowledge from 000–599
Volume 3
(J) Schedules: The organization of knowledge from 600–999
Volume 4
(K) Relative Index: An alphabetical list of subjects with the disciplines in which they are treated subarranged alphabetically under each entry