Originally developed for use by the Library of Congress (LC), the Library of Congress Subject Headings (LCSH) is a set of headings that are authorized for use as subject access points in bibliographic records. LCSH is a general vocabulary, meaning it does not specialize in any particular disciple or domain. Most libraries in the USA and many across the world use the LCSH and their main subject-controlled vocabulary. 

​

The majority of the terms in the LCSH vocabulary are topical subject headings but other types, such as authorized access points for place, person, family, and corporate names can be used in the construction of subject headings.

​

The complete and most updated LCSH is available through the LC's Classification Web subscription tool. PDFs of the LCSH (not always the latest version) and LC authority records of established headings and subdivisions are available for free via the LC website (loc.gov). 

The structure or the organization of the vocabulary is in alphabetical order. Equivalence, hierarchical, and associative relationships between subjects are indicated with the use of cross reference, using the notations of UF (used for) and USE for the hierarchical relationships, BT (broader term) and NT (narrower term) for the hierarchical relationships, and RT (related term) for all associative relationships.

In LCSH authority records, these relationships are indicated by the respective MARC authority fields and relationship codes.

LCSH is a highly pre-coordinated vocabulary. Established combinations exist in the vocabulary but the system also allows the cataloger to construct numerous combinations of headings and subdivisions, following the Subject Headings Manual guidelines and instructions under existing main headings and subdivisions. This allows for the expression of very complex topics into one string but is also seen as a challenge for users when interacting with the library catalog.

As a pre-coordinated system, LCSH uses different types of subdivisions that can be attached to a main heading for the expression of complex topics. These types are reflecting different facts, such as topical, geographical, chronological, and form.

​

Free-floating subdivisions are a group of common subdivisions (topical & form) that may be used without prior authorization under a particular subject or name heading where applicable and appropriate.

Certain subdivisions, topical or form, are common in a particular subject field. These subdivisions are listed under a chosen heading in the category instead of repeating them under each heading within the same category. The chosen heading serves as a pattern heading of subdivisions for all other headings in that category.

The Subject Headings Manual (SHM) provides guidelines to use the LCSH. Originally developed as an in-house procedure manual, it became a tool for other libraries — in fact — most users of the SHM are now outside the LC. The manual includes explanations of subject cataloging policies, procedures, and practices in proposing new headings to be included in LCSH and assigning LCSH subject headings to bibliographic records for subject representation and access.

The Children’s Subject Headings include alternate headings for use with children’s materials. These headings are not in LCSH and are different in form from those found in LCSH.