Subject Heading (Multi-Level Vocabulary)

Definition, description and example of a subject heading

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Definition of a Subject Heading

A type of multilevel controlled vocabulary in which metadata values are classified into categories that may be broad classes.


Subject headings provide an organizational structure in which metadata values are grouped according to broad classes. This type of vocabulary provides a big picture scheme, where values can be viewed in the context of established, overarching subjects.

The classes in a subject heading are used to provide a contextual basis for each member; that is, the subject heading shows how a particular value fits into an established context. A particular value can be used in multiple contexts, so a value can be found in multiple subject headings.

For example, you might place the metadata value "water" in two subject headings--chemical or geographic. The chemical subject heading might include things like elements present (hydrogen and oxygen), physical state (solid, liquid, or gas), or chemical properties (universal solvent, etc.). The geographic subject heading might include bodies of water (lake, river, ocean), extreme weather (flood, draught), or water source (snow, aquifer, purification plant).

Note: While subject headings tend to include broader classes, taxonomies, the other multilevel controlled vocabulary discussed in this section of the guides, tend to include subject-specific, narrow classes.


One of the most-used subject headings is managed by the Library of Congress. The LCSH (Library of Congress Subject Headings) is a vocabulary that is used to catalog printed materials available in a library setting. In the LCSH, printed media about human beings might be included in one of a variety of classes, as shown below.

Compare this to the Linnaean classification for a human being that was described in the Taxonomies guide. In that vocabulary, the Homo sapiens label uniquely identified human beings, rather than including them in several classes.

  • Library of Congress Classification Outline
  • Class Q - Science
  • Subclass QH: Natural history - Biology
  • QH301-705.5: Biology (General)
  • QH359-425: Evolution
  • QH426-470: Genetics
  • QH471-489: Reproduction
  • QH501-531: Life
  • QH540-549.5: Ecology
  • QH573-671: Cytology
  • QH705-705.5: Economic Biology
  • Subclass QM: Human anatomy
  • QM1-511: General
  • QM531-549: Regional anatomy
  • QM550-577.8: Human and comparative histology
  • QM601-695: Human embryology
  • Subclass QP: Physiology
  • QP1-345: General, including influence of the environment
  • QP351-495: Neurophysiology and neuropsychology
  • QP901-981: Experimental pharmacology
  • Subclass QR: Microbiology
  • QR1-74.5: General
  • QR180-189.5: Immunology
  • QR355-502: Virology

Notice this subject heading does not uniquely identify an object; it merely classifies it according to "like objects." The classification scheme provides us with general information about an object. Since classes in subject headings are broad, the lower objects in the lower classes do not inherit a significant amount of characteristics. For example, if something is classified with the number QH525, we would know only that it is a scientific piece about life in natural history and biology.

Suggested Citation

Neiswender, C. 2011. "Subject Heading (Multi-Level Vocabulary)." In The MMI Guides: Navigating the World of Marine Metadata. Accessed August 15, 2020.