The Library of Congress classification system has 20 classes, as follows:

А    General works   
B-BJ    Philosophy   
BF    Psychology   
BL-BX    Religion   
C,D,E,F    History       
G    Geography,Anthropology,    Recreation
H    Social Sciences,Business   
J    Political Science   
KF    U.S. Law        Publishing
L    Education       
M    Music
N    Fine Art
P    Language & Literature
Q    Math, Science, Computer Science
R    Medicine
s    Agriculture
T    Technology, Engineering
U    Military Science
V    Naval Science

As you noticed, the letters do not necessarily stand for the first letter of the subject they represent. For instance, Political Science is letter J, and Fine Art is letter N.

A Library of Congress call number usually has three parts.
•    Part 1: one or two letters for the broad subject area.
•    Part 2: numbers and is a further subdivision of the gener¬al subject.
•    Part 3: a letter and number code for the author's name.