How Call Numbers Work


Before visiting a library, you should understand how library materials are organized.

Almost all materials (i.e. books, journals, audio cassettes, videotapes, and microfilm) housed in a library are assigned call numbers. Call numbers identify an item's subject and location in a library. Call numbers are assigned to items based on a classification scheme. Library materials on similar topics will have similar call numbers and will be shelved together.

There are three major classification systems used by most libraries in the United States--Library of Congress Classification System (LC), Dewey Decimal Classification System (DDC), and the Superintendent of Documents System(SuDocs). More information is provided on each of these systems below.


Library of Congress Classification System

Most college and university libraries use the Library of Congress (LC) Classification System. This system divides materials into 21 different subject areas. An LC call number consists of one or two letters followed by a number.

Below is a list of the LC subject groupings.

LC Call Number  Subject Area
A General Works
B Philosophy
C Auxiliary sciences of history
D History--Old World
E-F History--America
G Geography, Anthropology, Recreation
H Social Sciences
J Political Science
K Law
L Education
M Music and books on music
N Fine Arts
P Language and Literature
Q Science
R Medicine
S Agriculture
T Technology
U Military Science
V Naval Science
Z Bibliography, Library Science

Use the following guidelines to read call numbers: 

  1. Materials that are LC classified are shelved alphabetically (left to right) by the top letter or letters in the call number (B, BC, etc.). 
  2. All call numbers of the same top letter are arranged numerically by the number below (B351, BC2, BC10, etc.). This number is treated as a whole number.
  3. All materials with the same number are arranged first alphabetically by the letter after the decimal point; and then by the decimal number after the letter (.C3, .D3, .D31, .D4, etc.). 
  4. If another letter appears after the decimal number, it is also arranged first by letter and then by decimal number.

Using these guidelines, the following LC call numbers would be found in this order on the shelf, reading from left to right: 



Dewey Decimal Classification System

Dewey Decimal Classification System (DDC) is often used in public libraries or K-12 school libraries. The system divides materials into ten major classes which are further subdivided into subclasses. Decimals are used for further subdivisions. 

Below are the ten major classes in DDC:

Dewey Call Number Subject Area
000 Generalities 
100 Philosophy/Psychology
200 Religion
300 Social Sciences
400 Language
500 Natural Sciences/Mathematics
600 Technology
700 The Arts
800 Literature/Rhetoric
900 Geography/History

When reading DDC call numbers, use the following guidelines:

  1. The number before the decimal point is always treated as a whole number.
  2. The numbers following the decimal point are treated as decimals.
  3. If another letter appears after the decimal number, it is also arranged first by letter and then by decimal number.

Using these guidelines, the following DDC call numbers would be arranged left to right on the shelf:



Superintendent of Documents Classification System

The Superintendent of Documents Classification System (SuDocs) is often used by libraries to organize collections of government documents. The system was created by the U.S. Superintendent of Documents to classify government documents according to the government agency which published them. 

A SuDocs number begins with a one, two or three-letter code indicating a U.S. government agency. For example, A for Department of Agriculture, ED for Department of Education, and T for the Treasury Department. This letter is followed by a number indicating the sub-agency which issued the publication. This is followed by a colon, then a letter/number combination identifying the individual title, volume, year or issue number. 

All numbers in SuDocs are read as whole numbers. The following numbers are in correct sequence, left to right.

: Si 3/7
: F39
: W36/2/989