The third summary contains the thousand sections. The third digit in each three-digit number indicates the section. A decimal point follows the third digit in a class number, after which division by ten continues to the specific degree of classification needed. A subject may appear in more than one discipline. 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.
Sometimes, other devices must be used to express the hierarchy when it is not possible or desirable to do so through the notation. Relationships among topics that violate notational hierarchy are indicated by special types of headings, notes, and entries. A guide to classifying in difficult areas, information on new schedules, and an explanation of the policies and practices of the Decimal Classification Division at the Library of Congress. They are repeated at the top of each page where their subdivisions continue. Subordinate numbers appear in the number column, beginning with a decimal point, with the initial three digits understood.