Library of Congress Classification

The Library of Congress Classification System divides knowledge into 21 branches represented by letters:

A General works M Music
B Philosophy N Visual arts
C Auxiliary sciences of history P Language and literature
D History (general) Q Science
E-F History (Americas) R Medicine
G Geography S Agriculture
H Social sciences T Technology
J Political science U Military science
K Law V Naval science
L Education 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