Library of Congress call numbers group items by general subject. The following letters represent some of the important subject and sub categories in history and related fields:
E 11 - 143 America
E 151 - 889 United States
E 184 - 185.98 Elements in the population
E 184.5 - 185.98 African-Americans
F 1 - 975 United States local history
H 1 - 99 Social sciences (General)
HC 10 -1085 Economic history and conditions
HQ 1101 - 2030.7 Women. Feminism