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 660 - 738 Late nineteenth century, 1865 - 1900
E 740 - 837.7 Twentieth century
E 838 - 889 Later twentieth century, 1961 - 2000
F 1 - 975 United States local history