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NLM Classification 2017: Worldwide source of medical library classification

NLM Classification Practices

Basic Rules
Form Numbers
Table G (Geographic Notation)
        Nineteenth Century Titles
        Early Printed Books
Serial Publications
Library of Congress Classification Schedules
        Special Instructions
Changes in Classification Practices
        Numbered Congresses-W3, W3.5 and ZW3
        Subdivision by Language and Publication Date
        Nurses' Instruction


The Library applies subject classification primarily to materials treated as monographs. Serial publications are separated by form and are assigned classification numbers within several broad categories.

The classification practices outlined below are current conventions. They are provided as explanation, stating NLM's general classification approach using the National Library of Medicine's and the Library of Congress's schedules, rather than 'how to classify' instructions.


The classification number assigned to a work is determined by the main focus or subject content of the work.

A work dealing with several subjects that fall into different areas of the classification is classed by emphasis, or if emphasis is lacking, by the first subject treated in the work. However, if the work is about a drug, special form of therapy, diagnostic procedure, or environmental condition as a cause of disease, etc., and the disease is only mentioned in the context of the primary subject (the therapy, etc.), classify as always for the primary subject.

A work on a particular disease is classified with the disease, which in turn is classified with the organ or region chiefly affected, regardless of special emphasis on form of therapy or diagnostic procedure used.


Each schedule, as well as some sections within a schedule (e.g., WO 201-233.1), contains a group of form numbers, generally 1 through 39, that are used to classify material by publication type within the general subject area of the schedule. In general, classification by publication type takes precedence over classification by subject. For example, atlases illustrating the pathology of Liver Diseases are all classed in WI 17 along with atlases on any other subject covered by the WI schedule for Digestive Diseases.

Collected works by several authors or by individual authors, and works comprised of addresses, essays, and lectures are classed in their respective form numbers. With the 2017 summer edition, collected works and works comprised of addresses, essays, and lectures that cover a particular subject are also classed in their respective form numbers within the schedule. For example, Psychiatry - Collected Works is classified in WM 5 or WM 7. Psychotherapy - Collected Works is also classified in WM 5 or WM 7.

Some numbers in the range of 1 through 39 are not true form numbers, that is, they are used to classify material with a special emphasis, such as 18 for education or 33 for discussion of law.

In general, the form number 11 is used for works dealing with the history of any aspect of a subject within a classification schedule. For example, WM 11 is used for both Psychiatry - history and Psychotherapy - history. There are exceptions to the use of the form number for history, which are generally noted under the particular form number (e.g., WC 11, is annotated "classify history of a single infectious disease with the disease"). The history of schools, laboratories, hospitals, institutes, etc., is classed in the appropriate form number for the type of entity. Furthermore, the form number 11 is not assigned to the schedules W and WB. The history of health professions and the practice of medicine are instead classified in the WZ schedule, except where otherwise annotated.

TABLE G (Geographic Notation

Geographic subdivision is provided for certain subjects in the NLM schedules by the application of Table G.

The use of geographical breakdown is restricted to those classes which are annotated with "Table G" in the schedules and includes both monographs and serials.

If a work on a subject that is geographically subdivided covers an area larger than the entity represented in a Table G notation it is classified in the General coverage (Not Table G) number, directly following the class number that provides for geographic subdivision. For example WG 11 History (Table G) is the number for the history of cardiology in particular geographic areas and WG 11.1 General coverage (Not Table G) is the number for books with general coverage of the history of cardiology.


Several types of monographic publications are classified according to special plans: Nineteenth century titles, Early printed books, and Bibliographies. Classification numbers for these publications do not appear in the Index.

Nineteenth Century Titles

A simplified subject classification derived from the letters that represent the preclinical and clinical subjects covered by the NLM Classification is used for nineteenth century (1801-1913) monographs. This abbreviated classification is limited to combinations of letters and the classification notations W1-6, W 600, WX 2 and the form number 22 that appears throughout the schedule. In addition, the entire WZ schedule, History of Medicine, is used for nineteenth century titles. When the subject falls outside of the schedules of the NLM Classification , only the letters of the LC schedule representing the subject are used, e.g., BF Psychology, SF Veterinary Medicine, etc. Facsimiles and reprints of entire nineteenth century works are classified in the 19th Century Schedule. Bibliographies imprinted in the nineteenth century use the special plan for Bibliographies rather than the 19th Century Schedule.

Early Printed Books

Works published before 1801 and Americana, i.e., early imprints from North, South and Central America and the Caribbean islands, are considered early printed books and are classified in the WZ schedule, WZ 230-270. These books are arranged alphabetically by author within each century or in the Americana number. (See WZ 270 for specific guidance by state for the coverage of Americana.) Reprints and translations of pre-1801 works are classified in WZ 290-292, and modern criticism of early works in WZ 294.


A bibliography within the scope of the NLM Classification is classified in the number for the subject, prefixed by a capital Z. Bibliographies outside the scope of the NLM Classification are classed in LC's Z schedule for Bibliography. Numbers for bibliographies are seldom given in the Index but are derived by using the instructions below for formulating the call number of a bibliography. Unless otherwise noted, the classification numbers for bibliographies may be used for both monographs and serials.

General medical serials ZW 1
General medical serials in one library ZW 1
General medical monographs and/or serials issued periodically ZW 1
Monographic works on general medicine ZWB100
  (monographs only)
General holdings of libraries in special fields (including private libraries) [Not LC practice]
    Chiropractic Z675.C48
    Dentistry Z675.D3
    Hospital Z675.H7
    Medicine Z675.M4
    Mental health Z675.M43
    Nursing Z675.N8
    Occupational health Z675.O22
    Pharmacy Z675.P48
    Veterinary medicine Z675.V47
    Others, A-Z as listed in LC's Z schedule under Z 675  
General monographic holdings of non-specialized libraries, Z881-977
    university, public, etc., by country  
General serials holdings of non-specialized libraries and union lists Z6945
    of serials  
Specific topics in medicine and allied fields Z+ NLM schedule letters
Specific topics in fields outside scope of NLM classification
    Exception: ZQ 1 is used for bibliography of general
    scientific periodicals and ZSF [and number] for subjects
    in the SF schedules.
    Other exceptions made in the past will no longer be used.
Z 5051-7999
General materials published in a particular country (national Z 1201-4980
General serials published in a particular country Z 6947-6964
Private library catalogs, other than those in Z 675 Z997
Booksellers catalogs  
    Monographs Z998-1000.5
    Serials Z6946-6964
    General Z5053-5055
    Of schools of dentistry, medicine, nursing, pharmacy,
    public health, veterinary medicine, etc.
            Individual (with the university) W4
            Collective ZW4
        United States (by subject) ZSF, ZQS-ZWZ
General bibliographies of periodicals Z6941


NLM follows the Anglo-American Cataloguing Rules, second edition, revised 2002, in defining serials. A serial is a "... continuing resource issued in a succession of discrete parts, usually bearing numbering, that has no predetermined conclusion. ... Serials include journals, magazines, electronic journals, continuing directories, annual reports, newspapers, and monographic series."

Serials are classified in the form number W1 with the exceptions noted below.


Government Administrative Reports or Statistics (W2)
Serial government publications that are administrative or statistical in nature are classed in W 2. Integrated reports of administrative and/or statistical information on several hospitals under government administration are classed in W 2. Serials classified in W 2 are sub-arranged by jurisdiction according to Table G.

Hospital Administrative Reports or Statistics (WX 2)
Serial hospital publications that are administrative or statistical in nature, including reports of single government hospitals, are classed in WX 2. Serials classified in WX 2 are sub-arranged geographically according to Table G.

Directory, Handbooks, etc.
Certain publication types, such as directories, handbooks, etc., issued serially are classed in form numbers used also for monographs. For example, directories, whether monographic or serial in nature, are classed for the publication type Directory in form number 22. Numbers used for both types of publications are identified in the schedules with the parenthetical note "(Used for both monographs and serials)". The appropriate LC schedule is used for the above defined publication types when their subject falls outside the scope of the NLM Classification.

Bibliographies and Indexes
Serial publications of bibliographies or indexes are classed according to the instructions in the section on Bibliographies above.


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. Otherwise, the Library of Congress schedules augment the NLM Classification for subjects related to medicine. NLM rarely uses LC's schedule for Law (K) except for general works. Legal works related to medicine are classified with the subject rather than the law.

Some LC class numbers are provided in the Index to the NLM Classification. Although these numbers are verified against the LC schedules periodically, the pertinent LC schedules must be consulted since the numbers may change over time. Hyperlinks are not provided to LC class numbers.

Special Instructions

Below are listed those LC schedules with special instructions for subjects that fall within both the NLM and LC schedules.

QD - Chemistry -- Use QU or QV if any portion of a work is devoted to biochemistry or pharmacology.

QH - Natural Sciences (General) -- Classify here general works on biology, ecology, and evolution, when they do not include biomedical content.

QK - Botany -- Use QV 766-770 if any portion of the work is devoted to medicinal plants; or WB 925 -- Herbal medicine, if the emphasis is on drugs derived from plants.

QL - Zoology -- Classify here non-pathogenic invertebrates. Pathogenic invertebrates are classed in NLM's QX schedule.
Vertebrates -- Anatomy and physiology of domestic animals are classed in SF (see below). Care and clinical use of laboratory animals in QY 50-60. Works on experimental studies in the interest of learning more about human disease are classed in the appropriate NLM schedule numbers.

QP - Physiology -- Classify here only physiology of wild animals in general.
Physiology of domestic animals is classed in SF. Special topics in this area, when applicable to humans, are classed in the appropriate NLM numbers; for example, Altitude, WD 710-715, Body temperature regulation, QT 165.

SF - Animal culture -- Classify here anatomy and physiology of domestic animals.

T - Technology -- Classify here Human engineering TA, Biotechnology TP; however, works
on Biomedical engineering are classed in NLM's QT schedule.

U - Military Science -- Classify here administrative documents pertaining to military hospitals and services.


Numbered Congresses -- W3, W 3.5 and ZW 3

NLM discontinued classifying serial publications of congresses or sequentially issued, numbered and dated monographic congresses in W3 in 1988. All newly acquired monographic congresses, including those of named meetings previously classified in W3, are classed in the appropriate subject classification number. Newly acquired serial publications that are proceedings or reports of meetings are classified in W1. Analytics of serials classified in W3 continue to be classified in the original W3 call number.

Subdivision by Language and Publication Date

For a brief period, with publication of the NLM Classification, 4th edition, 1978, certain classification numbers were subdivided by language and date. Throughout the schedule, classification numbers that were heavily used were categorized as materials published earlier than 1978 and from 1978 forward and were further modified by appending a .3 (for English language) or .4 (for foreign language).


WG 140 Electrocardiography. Echocardiography. Monitoring (General)
    WG 140.3 English language, 1978-
    WG 140.4 Other languages, 1978-

This practice was discontinued in 1980 and the numbers dropped from the NLM Classification, 4th ed., rev., published in 1981. Materials which were already classified in these numbers were not reclassified.

These canceled numbers are not included in the cumulative list of “Canceled Class Numbers.”.

Nurses’ instruction

Since 1984 materials on specific subjects, prepared for nurses, have been classified with the subject, when the nurses' role is not discussed. For example, material on the physiopathology of hypertension, written for nurses, is classified with Hypertension in WG 340. However when nursing techniques are included the material is classified in WY. Prior to 1984 background materials were classified in the WY schedule together with materials dealing with nursing techniques in special fields of medicine.