Many academic libraries use the Library of Congress Subject Headings (LCSH) which are 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 Remote working: linking people and organizations has the call number HD 2336.3:

H - Social sciences and business

HD - Economic history and conditions

2321 - 4730 - Industry

2331 - 2336 - Household Industry

2336.3 - Telecommuting