title: National Library of Medicine classification
text: The National Library of Medicine (NLM) classification system is a library indexing system covering the fields of medicine and preclinical basic sciences. The NLM classification is patterned after the Library of Congress (LC) Classification system: alphabetical letters   denote broad subject categories which are subdivided by numbers. For example, QW 279 would indicate a book on an aspect of microbiology or immunology.
  
  The one- or two-letter alphabetical codes in the NLM classification use a limited range of letters: only QS–QZ and W–WZ. This allows the NLM system to co-exist with the larger LC coding scheme as neither of these ranges are used in the LC system. There are, however, three pre-existing codes in the LC system which overlap with the NLM: Human Anatomy (QM), Microbiology (QR), and Medicine (R). To avoid further confusion, these three codes are not used in the NLM.
  
  The headings for the individual schedules (letters or letter pairs) are given in brief form (e.g., QW - Microbiology and Immunology; WG - Cardiovascular System) and together they provide an outline of the subjects covered by the NLM classification. Headings are interpreted broadly and include the physiological system, the specialties connected with them, the regions of the body chiefly concerned and subordinate related fields. The NLM system is hierarchical, and within each schedule, division by organ usually has priority. Each main schedule, as well as some sub-sections, begins with a group of form numbers ranging generally from 1–49 which  classify materials by publication type, e.g., dictionaries, atlases, laboratory manuals, etc.
  
  The main schedules QS-QZ, W-WY, and WZ (excluding the range WZ 220–270)  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.
  
  Overview of the NLM Classification categories
  
  Preclinical Sciences
   QS Human Anatomy
   QT Physiology
   QU Biochemistry
   QV Pharmacology
   QW Microbiology & Immunology
   QX Parasitology
   QY Clinical Pathology
   QZ Pathology
  
  Medicine and Related Subjects
  
   W  Health Professions
   WA Public Health
   WB Practice of Medicine
   WC Communicable Diseases
   WD Disorders of Systemic, Metabolic, or Environmental Origin, etc.
   WE Musculoskeletal System
   WF Respiratory System
   WG Cardiovascular System
   WH Hemic and Lymphatic Systems
   WI Digestive System
   WJ Urogenital System
   WK Endocrine System
   WL Nervous System
   WM Psychiatry
   WN Radiology. Diagnostic Imaging
   WO Surgery
   WP Gynecology
   WQ Obstetrics
   WR Dermatology
   WS Pediatrics
   WT Geriatrics. Chronic Disease
   WU Dentistry. Oral Surgery
   WV Otolaryngology
   WW Ophthalmology
   WX Hospitals & Other Health Facilities
   WY Nursing
   WZ History of Medicine
   19th Century Schedule
  
  See also
  Dewey Decimal Classification
  Colon Classification
  Library of Congress Classification
  Universal Decimal Classification
  
  References
  
  Citations
  
  Sources 
  
   
  
  Medical classification
  Knowledge representation
  Library cataloging and classification