"The NLM Classification covers the field of medicine and related sciences. The scheme is a system of classification intended to be used for the shelf arrangement of all library materials, regardless of format. Designed as a broad classification, it is suitable for both large and small library collections and may be adapted to handle specialized collections of any size. The NLM Classification is a system of mixed notation patterned after the Library of Congress (LC) Classification where alphabetical letters which denote broad subject categories are further subdivided by numbers. The NLM Classification utilizes schedules QS-QZ and W-WZ, permanently excluded from the LC Classification Schedules and is 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 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., dictionaries, atlases, laboratory manuals, etc. The main schedules QS-QZ, W-WY, and WZ (excluding WZ 220-270) are used to classify works published after 1913; the 19th century schedule is used for works published 1801-1913; and WZ 220-270 is used to provide century groupings for works published before 1801 and Americana. Geographic subdivision is provided for certain subjects in the NLM schedules by the application of Table G notations."