A -- General Works
B -- Philosophy, psychology, and religion
C -- History: Auxiliary Sciences
D -- History: General and Old World
E -- History: America and U.S., general
F -- History: America and U.S., local
G -- Geography, anthropology, folklore, etc.
H -- Social sciences, economics, business, sociology
J -- Political Sciences
K -- Law
L -- Education
M -- Music
N -- Fine arts
P -- Philology, linguistics, language, and literature
Q -- Science
R -- Medicine
S -- Agriculture
T -- Technology
U -- Military Science
V -- Naval science
Z -- Bibliography and library science
I, O, X, and Y are not used