Because a book is usually stored in one specific physical place, classification systems allow that each element is placed in one class only. However in some cases additional entries can be present together with main entries.

For simplicity we group the many classification systems in three categories: enumerative, hierarchical and faceted. Enumerative systems order the subject headings alphabetically and assign them numbers, hierarchical systems go from general subjects to more specific ones and faceted systems are more complicated with subjects divided in to facets which are mutually exclusive. Many systems of classification contain in same proportion the three types of classification.

The most common systems used in English-speaking countries are the BC (Bliss bibliographic classification), the DDC (Dewey decimal classification) and LC (Library of Congress classification).

New classification systems tend to be more based on the synthesis. They tend to combine attributes from different lists, to reflect the different aspects of a piece.

For more detailed resources on classification, the following sites provide useful information:
Stanford University Classification Resources - (palimpsest.stanford.edu/lex/)
Bliss Classification Association - (www.blissclassification.org.uk/)
Library Classification Schemes for Internet Resources - (staff.oclc.org/~vizine/Intercat/vizine-goetz.htm)