Library materials are shelved alphabetically and numerically depending on their call numbers, which are determined by the items' subjects.  Knowing the classification letter(s) for your subject area can help you locate materials on specific and related subjects, as well as browse the shelves for "fortunate finds."

For an overview of the Library of Congress classification system,
see the LC Classification Outline, which shows the letters and titles of main classification classes and is offered online by the Library of Congress Cataloging Policy and Support Office.
 
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FORMATS USED TO WRITE CALL NUMBERS
Call numbers on library materials and in the catalog appear in two formats and are read from top to bottom or left to right, as shown below:
 
Stacked on the book spine
""Z
""678.9
""H9
""1987
 		 In a single line
in the online catalog record

Z678.9 H9 1987
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HOW TO READ CALL NUMBERS
When reading call numbers, consider each segment before moving on to the next segment. This process is illustrated in the following tables, which explain and give examples of the segments in Library of Congress call numbers. In each row, call numbers are arranged in correct shelf order to show how they would be arranged on library shelves. Bold type indicates the portion of the call number being discussed.
 
Letter(s) comprise the first segment and indicate an item's subject and sub-subject.
The library's arrangement is based on these letters, so it's primarily alphabetic.		 H
41
E561		 HA
17
K4
1971		 HB
34
N6
1990		 HC
240
C518
2000
 
Numbers comprise the next segment and more specifically indicate subject.
Read these numbers as whole numbers. If this component contains a period and another number (i.e. 674.7 and 678.93), read it like you would a whole number with a decimal.		 Z
665
M672
1984		 Z
674
K35		 Z
674.7
L52
1990

 		 Z
678.93
M53
A54
1987
 
Letters and numbers comprise the next segment and indicate an author's last name, a subject subdivision or, sometimes, title.

Read the letters alphabetically.


Read the numbers after the letter as decimals.
KF
387
C53
 

M466
KF
387
H36
 

M5
KF
387
K55
 

M84		 KF
387
L37

M9

 

Another segment of letters and numbers may follow the previous one.
Again, read the letters alphabetically and the numbers as decimals.
 
The last segment, if given, indicates the item's publication date.

Items with the same call number are shelved chronologically by publication date.		 PS
3515
U274
A6
1959
 		 PS
3515
U274
A6
1969
 		 PS
3515
U274
A6
1971
 		 PS
3515
U274
A6
2000

(Adapted from a page located at Tarleton State University, Stephenville, TX)