Many academic libraries use the Library of Congress (LC) Classification System which is divided into 21 branches of knowledge represented 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

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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