The National Library of Medicine Classification covers the field of medicine and related sciences. The scheme is a system of classification used for the shelf arrangement of all library materials, regardless of format.

The NLM Classification is a system of mixed notation patterned after the Library of Congress (LC) Classification where letters of the alphabet representing broad subject categories are subdivided by numbers. The NLM Classification utilizes schedules QS-QZ and W-WZ, which are permanently excluded from the LC Classification Schedules, and are intended to be used with the LC schedules which supplement the NLM Classification for subjects bordering on medicine and for general reference materials. The LC schedules for Human Anatomy (QM), Microbiology (QR), and Medicine (R) are not used at all by the National Library of Medicine since they overlap the NLM Classification.

The first edition of the Library's Classification was published in 1951 as the Army Medical Library Classification. It established the current structure of the classification as well as NLM's classification practices.

The headings for the individual schedules are given in brief form (e.g., WE - Musculoskeletal System; WG - Cardiovascular System) and together they provide an outline of the subjects that constitute the NLM Classification. These headings are interpreted broadly and include the physiological system, the specialty or specialties connected with them, the regions of the body chiefly concerned and subordinate related fields. The Classification is hierarchical, and within each schedule, division by organ usually has priority.

Each main schedule (as well as some sections within a schedule) begins with a group of
form numbers ranging generally from 1-49 which are used to classify materials by publication type, e.g., atlases = 17, dictionaries = 13, directories = 22, handbooks = 39, etc.
Textbooks are classed in the broadest subject area of each schedule.

• View an outline of the NLM classification schedule
  with links to selected Biomedical Library holdings.

• View an alphabetical subject index to the NLM classification schedule
  with links to selected Biomedical Library holdings.

LC Classification Schedules

This system was developed in the early 1900's as a utilitarian classification for the holdings of the Library of Congress (LC) and for what it expected to add in the future. The scheme was not based on any philosophical plan for the classification of knowledge, but rather on the relationship of subjects as represented in the collection of the Library of Congress. The classification is a general enumerative scheme consisting of 21 classes. The individual schedules were independently created by subject specialists and, though developed separately, each represents a unified part of an overall scheme. The main classes and subclasses are designated by one, two or three capital letters. Further subdivision is in numbers ranging from 1 to 9999, each of which may be extended further by either a decimal or an alphabetical A to Z arrangement. The lack of a truly hierarchical structure allows for infinite expansion within the various schedules.

• View an outline of the LC classification schedule.

• View an alphabetical subject index to the LC classification schedule.