A classification system developed and used at the Library of Congress since 1897, the Library of Congress Classification system (LC) divides the field of knowledge into twenty large classes with an additional class on general works. This notation allows more combinations and greater specificity without long call numbers. Many libraries on campus, including the Undergraduate Library and the Social Sciences, Health, and Education Library, use LC classification schemes for all or part of their collections. 

For more information see the Library of Congress's information page.

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

The Anatomy of a Call Number: Library of Congress

Subject

Indicates the subject matter of the book. A single letter represents a broad, general subject. For example, "K" represents "Law" Multiple letters represent a narrower subject within it. For example: "KF" represents "Federal Law"  The number in the middle further defines the subject. For example, "801" represent "general/comprehensive works in the subject area of law"

Note:  When looking at this number count as you usually would: 80 comes before 800 which comes before 8000.