Please don't ask about how this actually works, just know, in your confused and wayward heart, that the subject you're searching the stacks desperately for, in whichever library you're in, falls somewhere into this Byzantine scheme conceived by sick Victorian-era library people (unless its Dewey, in which cases its another twisted bit of taxonomy all together):
A-- General Works & Theses
AE-- Encyclopedias (General)
AI-- Indexes (General)
AY-- Yearbooks, Almanacs, Directories

B-- Philosophy, Psychology, Religion
B-BD-- Philosophy, Logic
BF-- Psychology
BH-BJ-- Aesthetics, Ethics
BL-BX-- Religion

C-- Auxiliary Sciences of History
CB-- History of Civilization and Culture (General)
CC-- Archaeology (General)
CT-- Biography (General)

D-- History: General and all areas except North & South America
DA-DR-- England and Europe
DS-- Asia
DT-- Africa
DU-- Oceania (South Seas)

E-- History: United States

F-- History: US Local History, Canada, South America

G-- Geography, Anthropology, Recreation
G-GF-- Geography, Oceanography
GN-GT-- Anthropology, Prehistoric Archaeology, Folklore
GV-- Recreation, Physical Education, Dance

H-- Social Sciences (General)
HB-HC-- Economics
HD-- Management, Land Use, Industry, Labor
HE-- Transportation, Broadcasting
HF-- Commerce, Marketing, Accounting
HG-- Finance, Insurance, Investments
HJ-- Government Finance
HM-HX-- Sociology

J-- Political Sciences
J-- Official Documents
JC-- Political Theory
JF-- Constitutional History and Administration
JK-JO-- Government of Countries
JS-- Local Government
JX-- International Law

K-- Law
KF-- Law of the United States

L-- Education
LB-LC-- Theory and Practice of Education
LT-- Teaching Materials: grades K-12

M-- Music
M-- Music Scores
ML-- Literature of Music
MT-- Music Instruction and Study

N-- Fine Arts
NA-- Architecture
NB-NK-- Sculpture, Drawing, Painting, Printmaking
NK-- Decorative Arts, Crafts, Decoration and Ornament

P-- Language and Literature
P-- Philology and Linguistics
PA-- Classical Language and Literature
PB-PH-- European Languages
PJ-PL-- Oriental and Asian Language and Literature
PM-- American Indian Language and Literature
PN-- Literary History and Collections (Includes all the performing arts)
PR-- English Literature
PS-- American Literature
PT-- Germanic Literature

Q-- Science (General)
QA-- Mathematics, Computer Science
QB-- Astronomy
QC-- Physics
QD-- Chemistry
QE-- Geology
QH-- Natural History]
QK-- Botany
QL-- Zoology
QM-- Human Anatomy
QP-- Physiology
QR-- Microbiology

R-- Medicine

S-- Agriculture (Plant and Animal Husbandry)

T-- Technology (TK Computing & Software)
TA-TN-- Engineering (Structural, Civic, Mechanical, Aeronautics (TL), etc.)

U-- Military Science

V-- Naval Science

Z-- History of Books, Bibliography, Library Science