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MARC 21 Classification
June 2021

The five MARC 21 communication formats, MARC 21 Format for Authority Data, MARC 21 Format for Bibliographic Data, MARC 21 Format for Holdings Data, MARC 21 Format for Classification Data, and MARC 21 Format for Community Information, are widely used standards for the representation and exchange of bibliographic, authority, holdings, classification, and community information data in machine-readable form.

A MARC record is composed of three elements: the record structure, the content designation, and the data content of the record. The record structure is an implementation of the international standard Format for Information Exchange (ISO 2709) and its American counterpart, Bibliographic Information Interchange (ANSI/NISO Z39.2). The content designation--the codes and conventions established explicitly to identify and further characterize the data elements within a record and to support the manipulation of that data--is defined by each of the MARC formats. The content of the data elements that comprise a MARC record is usually defined by standards outside the formats. Examples are the Library of Congress Classification (LCC), the Dewey Decimal Classification (DCC), and other classification schemes used by the organization that creates a record. The content of certain coded data elements is defined in the MARC formats (e.g., the Leader, field 00, subfield $w in the 4XX and 5XX tracing fields).

The MARC 21 Format for Classification Data: Including Guidelines for Content Designation defines the codes and conventions (tags, indicators, subfield codes, and coded values) that identify the data elements in MARC classification records. This document is intended for the use of personnel involved in the creation and maintenance of classification records, in the publication of classification schemes from machine-readable data, as well as those involved in the design and maintenance of systems for the communication and processing of classification records. A section in the documentation entitled scheme-specific conventions describes coding practices for the two major classification schemes, the Library of Congress Classification and the Dewey Decimal Classification.

Classification data may be used for online public catalog retrieval systems, online systems for library classifiers (e.g., for machine-assisted classification), systems for the maintenance and development of classification schedules, validation of classification numbers assigned to bibliographic records, and linking to MARC authority records. Wherever possible, classification data elements were designed to be generic, i.e., usable for any classification scheme. Data element features were designed to accommodate the two major classification schemes in use in the United States: Dewey Decimal Classification (DDC) and Library of Congress Classification (LCC). This documentation is also available online, including a concise version and a simple field list at:


The MARC 21 Format for Classification Data is designed to be a carrier for information about classification numbers and captions associated with them. The source of the information is the classification scheme identified in field 084 (Classification Scheme and Edition), subfield $a (Classification scheme code).

Kinds of Classification Records

MARC classification records are distinguished from all other types of MARC records by the presence of code w (Classification data) in Leader/06, Type of record. The MARC 21 Format for Classification Data further identifies three kinds of classification records in 008/06, Kind of record:

Types of Numbers

The MARC classification record is an authority record for the classification number(s) and caption contained in field 153 (Classification Number). Classification numbers with captions may also appear in fields 453 (Invalid Number Tracing) and 553 (Valid Number Tracing). Index term only records contain an index term in field 154 (General Explanatory Index Term) instead of a classification number and caption in field 153. Three types of classification numbers are defined in the classification format and identified in 008/07 (Type of classification number):

Classification Number Usage in Bibliographic and Authority Records

MARC bibliographic records may contain classification numbers in fields 050-09X. MARC authority records may contain classification numbers in fields 053 (LC Classification Number) and 083 (Dewey Decimal Classification Number). Classification numbers may be valid, invalid or, in the case of a span, partially valid. A valid classification number may be used in a bibliographic or authority record by itself or with additional letters or numbers. In the case of a span, a portion of it may be used in a bibliographic record. In addition a number that has been synthesized by correctly applying a table, internal subarrangement or add instructions to a classification number or span is considered valid for use in a bibliographic or authority record; a classification record may or may not be created for synthesized numbers or numbers within spans.

A partially valid classification number span is one in which only the beginning or ending number of a span is valid. Only the valid portion may be used in bibliographic or authority records. An invalid number may not be used in a bibliographic or authority record. However, a classification record may be created for an invalid number in cases where the reference information cannot be communicated by a simple 453 tracing because a more detailed reference note is required. Invalid numbers in a 153 field of an invalid record or in a 453 field (Invalid Number Tracing) may not be used in bibliographic or authority records.

Required Classification Data Fields

A classification data record should contain at a minimum the following fields:

008 Fixed-Length Data Elements
084 Classification Scheme and Edition
153 Classification Number
154 General Explanatory Index Term


Description of Record Parts

A MARC classification record consists of three main components: the Leader, the Directory, and the Variable Fields. The following information summarizes the structure of a MARC record. More detail is provided in More detail is provided in MARC 21 Specifications for Record Structure, Character Sets, and Exchange Media.

There are two types of variable fields:

The variable data fields are grouped into blocks according to the first character of the tag, which identifies the function of the data within the record. The type of information in the field is identified by the remainder of the tag.

0XX Control information, numbers, codes
1XX Classification numbers and terms
2XX Complex see references
3XX Complex see also references
4XX Invalid number tracings
5XX Valid number tracings
6XX Note fields
70X-75X Linking entries
76X Number building fields

In the fields that contain classification numbers, references and tracings, the final two characters of the tag are the same. (153, 253, 353, 453, 553)

153 Classification Number
253 Complex See Reference
353 Complex See Also Reference
453 Invalid Number Tracing
553 Valid Number Tracing

Within the 1XX, 4XX, 5XX blocks, certain parallels of content designation are usually preserved in the subfield coding.

Within variable data fields, the following two kinds of content designation are used:

Multiscript Classification Records

A MARC classification record may contain data in multiple scripts. One script may be considered the primary script of the data content of the record, even though other scripts are also used for data content. (Note: ASCII is used for the structure elements of the record, with most coded data also specified within the ASCII range of characters). General models for multiscript data are described in Appendix C, along with several full record examples.

Field and Subfield Repeatability

Theoretically, all fields and subfields may be repeated. The nature of the data, however, often precludes repetition. For example, a classification record may contain only one 1XX field. The repeatability or non-repeatability of each field and subfield is specified in the format.

Fill Character and Related Values

A fill character (ASCII 7C hex), represented in this document as a vertical bar (|), may be used in classification records in field 008, and in control subfield $w of the 453 and 553 fields (Invalid and valid Number Tracing). A fill character may not be used in the Leader, or in tags, indicators, or subfield codes. The use of the fill character in a classification record indicates that the format specifies a code to be used, but the creator of the record has decided not to attempt to supply a code.

Code n (Not applicable) is defined in many coded positions to indicate that the characteristic defined by the position is not applicable to a specific type of number or kind of record.

Display Constants

A display constant is a term, phrase, spacing, or punctuation convention that may be system generated under prescribed circumstances in order to make a visual presentation of data in a record more meaningful to a user. In the classification format, certain field tags, subfield codes (e.g., subfields $c, $z), indicator values (e.g., field 684, Auxiliary Instruction Note, first indicator, type of note), and coded values (e.g., control subfield $w/0, Special relationship) may be used to generate specific terms, phrases, and/or spacing or punctuation conventions for the display of a record. The use of display constants is determined by each organization or system. Examples of display constants are provided under Scheme-specific conventions and Input Conventions in the field descriptions.

Record Content Responsibility

The MARC 21 Format for Classification Data serves as a vehicle for classification data of all types from any organization. In general, the responsibility for the data content, content designation, and transcription of data within a MARC record may be determined by examination of the field indicated in the responsible parties section below.

Responsible Parties

In unmodified records, the organization identified as the original cataloging source in 040 subfield $a is responsible for the content of the record. The organization identified as the transcribing agency in field 040 $c is responsible for the content designation and transcription of the data.

Certain data elements are defined in relation to the authoritative agency that maintains the list or rules upon which the data is based (e.g., field 750, Index Term−Topical). An organization using an authoritative-agency data element must use the authoritative list or rules specified for the data element. These lists or rules are specifically identified in the data element descriptions. The responsibility for the content of the data element belongs to the organization identified in subfields $a and $d of field 040.

Other data elements are defined in the format as being exclusively assigned by a single agency (e.g., field 010, Library of Congress Control Number). The content of agency-assigned elements is always the responsibility of the agency. The content designation and transcription of the data may be the responsibility of another organization identified in subfield $c of field 040.


Main Parts

MARC 21 Format for Classification Data consists of a Summary Statement of Content Designators followed by a detailed presentation of each content designator. Following the descriptions of the Leader and the Directory, the variable control fields are arranged in field tag order (001-008). The variable data fields are presented in the following groupings in order to bring together fields that serve related functions:

Appendixes to this document provide information about several control subfields (Appendix A), full level record examples (Appendix B), multiscript record examples (Appendix C), a glossary of terms used in the document (Appendix D), lists of changes to the format since the last edition or update (Appendix E), and a list of organization code sources (Appendix F).

General Information Sections

To avoid repetition, general information sections are provided for groups of fields with similar characteristics. These general information sections provide instructions for the content designators that are common to each field in the group. The description for each of the fields refers back to the general information section for that group.

A X53 General Information section is provided for Classification Numbers. This general information section provides instructions for the content designators that are common to classification numbers whether they are used in a 153 classification number field or in a 453 or 553 tracing field. The description for each of the classification number fields refers back to the X53 general information section.

A Tracing and Reference Fields−General Information section describes the use of the various reference note fields and tracings in constructing displays of cross references from these fields. It also provides instructions for applying subfields $i, $t and $w in the 453 and 553 tracing fields. The description for each of the reference note fields contains the detailed instructions for the content designators for the field. The user is referred to the Tracing and Reference Fields General Information section only for cross reference display descriptions. The 453 and 553 descriptions refer to both the X53 and the Tracings and References General Information sections for content designator instructions.

In addition, the format includes several general information sections to summarize blocks of fields. These are: 00X (Control Fields), 008 (Fixed-Length Data Elements), 1XX (Classification Numbers and Terms), 01X-08X (Numbers and Codes), 6XX (Note Fields), 70X-75X (Index Term Fields), and 76X (Number Building Fields).

Components of the Detailed Descriptions

A detailed description may consist of six parts: content designator listing; character position or field definition and scope; guidelines for applying content designators, with examples; input conventions; and content designator history.

The content designator listing area presents

For the Leader and the variable control fields, the list gives the name of the character position and any defined coded value. For the variable data fields, the list gives the name of the indicator positions and any defined coded values and the name of the subfield codes and any defined coded values. For all variable fields, the repeatability/nonrepeatability code at the field level specifies whether the field may be repeated in a record. For variable data fields, the subfield repeatability code specifies whether the subfield code may be repeated within a field.

The character position or field definition and scope area describes the contents of the character position or field, the appropriateness of its use in specific kinds of classification records, and gives other information, such as field repeatability in particular circumstances, the use of required lists or rules in formulating the data, etc.

The guidelines for applying content designators area describes the content and scope of each coded value, indicator, or subfield code and gives other information, such as appropriateness for use and repeatability in certain circumstances. The coded values and subfield codes in this area are presented in alphabetical/numerical order. Examples showing the use of the content designator are provided immediately following the description. The examples illustrate the application of specific MARC content designation. The data may be fuller or less full than would be used in actual cataloging practice. In some cases, policy decisions will need to be made concerning the application of MARC content designation. In addition, further editing, articularly of the LCC schedules, may be necessary to encode the data beyond what was done in this document..

The scheme-specific conventions area describes inputting practice applicable only to specific classification schemes, as identified by the classification scheme code in field 084 (Classification Scheme and Edition). Examples include the equivalents of scheme-specific terminology in MARC, display constants, and input conventions only applicable to specific schemes.

The input conventions area provides general guidance for the application of the content designators, and for such things as punctuation, spacing, and the use of display constants.

The content designator history area provides a record of significant content designator changes. The types of changes that are included are:

In the MARC formats, a distinction is made between obsolete and deleted content designators. An obsolete content designator is one that may have been used in MARC records and that may continue to appear in records created prior to the date it was made obsolete. Obsolete content designators are not used in new records. An obsolete content designator is recorded in the Content Designator History area. Content designation instructions are provided for retrospective conversion of records having data elements that would have been identified by the obsolete content designator. A deleted content designator is not recorded in the history area and will no longer appear anywhere in the MARC documentation. A content designator that had been reserved in MARC but has not been defined, or one that had been defined but is known with near certainty not to have been used, may be deleted from the format. A deleted content designator is available for redefinition in a format. A deleted content designator is available for redefinition in a format.

Typographical Conventions

Throughout this document, the following typographical conventions are used:

Link disclaimer


The MARC 21 Format for Classification Data should be used with the following standards and related documentation. When a standard is applicable to data in specific fields of the format, the fields are given in brackets following the citation.

Classification Schemes

National and International Standards:

ISO publications may be obtained from the International Organization for Standardization (ISO) and their agents; and ANSI/NISO Z39 publications may be obtained from the National Information Standards Organization (NISO).

MARC Standards and other related publications:

These publications are available from the Library of Congress, Cataloging Distribution Service, Washington, DC 20541, USA (Worldwide distribution). Where indicated these publications are available on the Internet.


The MARC 21 Format for Classification Data is prepared by the Network Development and MARC Standards Office, Library of Congress, in cooperation with Content Management Division, Library and Archives Canada (previously National Library of Canada) and Bibliographic Standards and Systems, British Library. Please direct any questions related to the content of this document to one of the following:

Network Development and MARC Standards Office
Library of Congress
101 Independence Avenue, S.E.
Washington, DC 20540-4402, USA
Fax: +1-202-707-0115
Email: [email protected]

Description Division
Library and Archives Canada
395 Wellington St.
Ottawa, Ontario K1A 0N4, Canada
Fax: +1-819-934-4388
Email: [email protected]

Collection Metadata
The British Library
Boston Spa, Wetherby
West Yorkshire LS23 7BQ, United Kingdom
Fax: +44 (0) 1937 546586
Email: [email protected]

The MARC 21 Format for Classification Data is organized on a field-by-field basis with each field separately paged to facilitate the updating of fields. Periodic updates of new and replacement pages for the base text and cumulated versions of the base text will be available from the Cataloging Distribution Service, Library of Congress, Washington, DC 20541-5017, USA (Worldwide distribution) and available in Canada from Publishing and Depository Services, Public Works and Government Services Canada (PWGSC), Ottawa, Ontario K1A 0S5, Canada (Canadian distribution). Their availability will be announced on the Library of Congress MARC website, the Library and Archives Canada MARC website, on listservs, and through press releases to the library press and to those who purchase the initial base volume. Updates are also available on standing order from the Library of Congress and the Canadian Government Publishing-PWGSC. This publication and all updates are supplied to all purchasers of the Library of Congress MARC Distribution Service files of classification records as part of their MARC record subscription.

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