Written and maintained by CASRAI Editorial Board
Last updated
A Library of Congress classification number is the letters-and-numbers class mark that places a work in one of the 21 classes of the Library of Congress Classification (LCC). Combined with a Cutter (book) number it forms the call number that decides where the item sits on the shelf and next to what. Researchers use it to browse adjacent works and to move from one known book to the rest of its subject area.
What a Library of Congress classification number is
The Library of Congress Classification (LCC, also searched as “LC classification” or “LCC classification”) was developed in the late nineteenth and early twentieth centuries to arrange the Library of Congress book collections. Other libraries later adopted it, especially large US academic libraries. The Library’s Policy, Training, and Cooperative Programs Division maintains it (Library of Congress, LCC overview, read 2026-11-07).
The system divides knowledge into twenty-one basic classes, each a single letter. Most classes divide into subclasses of two letters, occasionally three: class N, Art, has NA Architecture, NB Sculpture and ND Painting, among others. Inside a subclass, each topic (a “caption”) gets a single number or a span of numbers. Whole numbers run from one to four digits and can be extended with decimals. That letters-plus-number string is the classification number.
The 21 classes
The Library of Congress outline lists these main classes (LC Classification Outline, read 2026-11-07). Letters I, O, W, X and Y are not used as main classes.
| Letter | Class title (as listed by LC) |
|---|---|
| A | General Works |
| B | Philosophy. Psychology. Religion |
| C | Auxiliary Sciences of History |
| D | World History and History of Europe, Asia, Africa, Australia, New Zealand, etc. |
| E, F | History of the Americas (both letters carry this title in the outline) |
| G | Geography. Anthropology. Recreation |
| H | Social Sciences |
| J | Political Science |
| K | Law |
| L | Education |
| M | Music and Books on Music |
| N | Fine Arts |
| P | Language and Literature |
| Q | Science |
| R | Medicine |
| S | Agriculture |
| T | Technology |
| U | Military Science |
| V | Naval Science |
| Z | Bibliography. Library Science. Information Resources (General) |
How to read an LC call number, left to right
A call number is the classification number plus the part that separates one work from others in the same class. LC calls that second part the book number, represented by the Cutter or part of it (Classification and Shelflisting Manual, G 63 Cutter Numbers, July 2013, read 2026-11-07). The example below is a constructed pattern to show the parts, not a real catalogue record.
| Part | Constructed example | What it tells you |
|---|---|---|
| Class letter(s) | ND | Class N (Fine Arts), subclass ND (Painting) |
| Caption number | 1234.5 | A whole number of one to four digits, here extended with a decimal, marking a topic within the subclass |
| Letter-plus-numeral decimal | .B72 | An alphabetical subtopic list entry, written as a decimal (LC’s own examples are .B72 and .K535) |
| Book number (Cutter) | .K535 | An alphabetical mark that orders works within the same class number |
Libraries commonly add a publication year after the Cutter to order editions; the LCC overview does not define that element, so treat it as a local shelflisting convention when you see it.
Why the order is not plain alphabetical or text order
The class number is a number and can be extended with decimals, so read it numerically. The Cutter is also read as a decimal. LC’s Cutter instruction says Cutters are treated as decimal numbers so that expansion between any two is unlimited, and gives this filing order: .B3, .B47, .B56, .B583, .B66, .B7. Read digit by digit after the decimal point, .B47 comes before .B56, and .B583 falls between .B56 and .B66, even though .B583 is longer.
What a Cutter is
The term comes from Charles Ammi Cutter (1837-1903), who devised the Two-Figure Author Table for arranging books by author within a class. LC adapted his principles for its own collections, and in LC practice the Cutter gives the alphabetical arrangement on the shelf and in the shelflist (G 63). Per the same instruction, a Cutter may be built from the table or completed so that it files alphabetically among works already shelflisted, so a given surname does not always yield identical digits.
LCC compared with Dewey: where the hierarchy lives
LC states that relationships among topics in LCC are shown not by the numbers but by indenting subtopics under broader ones in the schedules, much like an outline. By contrast, in strictly hierarchical systems such as the Dewey Decimal Classification, hierarchy is shown by numbers that can be continuously subdivided (LCC overview). Practical consequence: to see where a number sits in the subject tree you need the schedule page, not just the digits.
Using classification numbers in research
- Browse by number. Each topic caption is assigned a number or span, so titles on the same topic share a number or sit nearby; browsing by call number can surface related titles that keyword searching misses.
- Work from a known item. Take the class letters and number of a core book and look up the surrounding span in the schedule to see broader, narrower and sibling topics.
- Expect local variation. Libraries adopt LCC for their own collections, so the call number on your library’s copy may be formed differently from the Library of Congress’s for the same book.
For other ways to find literature, see AI in literature search; for the field behind classification work, see what library science is; for lists of sources, see bibliography.
Where to look up LCC schedules
- Free outline and PDFs: the LC outline links each class, and the LCC overview says 44 print-ready PDF schedules are available on the web.
- Classification Web Plus: the Cataloging Distribution Service product that includes LCC and the text of Library of Congress Subject Headings.
- Full records and updates: LCC in MARC 21 or MARCXML, with weekly update subscriptions, from the same service.
Common misunderstandings
- “The number shows the hierarchy.” In LCC it does not; indentation in the schedule does.
- “The Cutter decimal is a quantity.” It encodes alphabetical filing position, not a measurement.
- “The call number is the classification number.” The call number also includes the book number.
FAQ
What is the difference between LC classification, LCC classification and a Library of Congress classification number?
The first two are the same system, the Library of Congress Classification. A classification number is one value within it: the class letters and number assigned to a topic.
How many classes does LCC have?
Twenty-one basic classes, each identified by a single letter (LCC overview).
Where can I find a Library of Congress classification number list or PDF?
The Library of Congress posts the class outline and 44 schedule PDFs on its site; the full text online is in the subscription Classification Web Plus.
Is LCC the same as Dewey?
No. Both organise library collections, but LCC uses letters and numbers with hierarchy shown by indentation, while Dewey shows hierarchy through its numbers.
Sources
- Library of Congress, Library of Congress Classification (page last updated 2023-12-19; read 2026-11-07).
- Library of Congress, Library of Congress Classification Outline (read 2026-11-07).
- Library of Congress, Classification and Shelflisting Manual, G 63 Cutter Numbers (July 2013; read 2026-11-07).
