Library of Congress Classification System

Most academic libraries in North America use the Library of Congress Classification Scheme, which divides all books into 21 broad branches of knowledge represesented by letters:

A - General works
B - Philosophy
C - Auxiliary sciences of history
D - History (general)
E-F - History (Americas)
G - Geography
H - Social sciences
J - Political science
K - Law
L - Education

 M - Music
N - Visual arts
P - Language and literature
Q - Science
R - Medicine
S - Agriculture
T - Technology
U - Military science
V - Naval science
Z - Bibliography; library science

When librarians determine the call number for a book they start with this breakdown. Additionally, each branch is divided into more specific topics represented by combinations of letters and numbers. For example, the book Teleworking has the call number HD 2336.3 B523 1995:
H - Social sciences and business
HD - Economic history and conditions
2321 - 4730 - Industry
2331 - 2336 - Household Industry
2336.3 - Telecommuting
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