Although it was developed to organize materials housed at the Library of Congress, the Library of Congress Classification system (LCC) is widely used in academic libraries around the world. The LCC system begins with 21 categories, each identified by a letter of the alphabet.
Most of the categories include subcatagories that are identified by 1 or 2 more letters. For example D contains World History and Histories of Europe, Asia, Africa, Australia, New Zealand, etc.; DS is Asia, a sub-topic in World History.
Individual topics are delineated by place, time period, etc. using numbers. For example, at DS801-897 you'll find books on Japan. A call number may have up to 4 digits (1-9999) in the main part, and then if necessary, decimal places that may have only numbers or both a letter and numbers.
Below are the major classes or categories found in LCC:
General Encyclopedias, Almanacs
Also includes Ethics, Logic, and Mythology
Archaeology, Genealogy, Heraldry, Numismatics (coins & currency)
Histories of Europe, Asia, Africa, Australia, New Zealand, etc.
USA, Canada, Latin America & the Caribbean
Also includes Folklore, Environmental Sciences, Leisure/Recreation, Oceanography
Business, Demography, Economics, Sociology, Statistics
Colonies/Colonization, Government, International Law, Political Institutions, Public Administration
Also includes Religious Law
All aspects of Education from history of to practice at all levels
Compositions, History and Literature of Music, Instruction and Study
Architecture, Decorative Arts, Design, Drawing and Painting, Sculpture, Visual and Print
Classical and Modern Languages, Literature, Poetry, Philology/Linguistics
Including all the Natural Sciences, Mathematics, Natural History
All Medical specialties, Homeopathy, Nursing, Pharmacy, Public Health
Aquaculture/Fishing, Forestry, Hunting, Plant Culture
Including Arts & Crafts/Handicrafts, Chemical Technology, Construction, Engineering, Home Economics, Metallurgy, Motor Vehicles, Photography
Armies: Organization, Maintenance, etc.
Marines, Navies: Organization, Navigation, Shipbuilding
Books: Industry & Trade, Information Resources, Libraries, Paleography, Writing