Design Principles for MARC 21  (Record Structure)

A MARC 21 format is a set of codes and content designators defined for encoding a particular type of machine-readable record.  The MARC 21 formats as a group serve as a vehicle for authority, bibliographic, classification, community information, and holdings data of all types.  These formats are intended to be communication formats and are primarily designed to provide specifications for the exchange of information between systems.  The following description of design principles repeats, in some cases, information given elsewhere in the Record Structure section, but is given again for completeness.

Content Designation:

The purpose of content designation is to identify and characterize the data elements which comprise a MARC record with sufficient precision to support manipulation of the data for a variety of functions.  The MARC 21 formats have attempted to preserve consistency of content designation across formats where this is appropriate.

The MARC 21 content designation supports the sorting of data only to a limited extent.  In general, sorting must be accomplished through the application of external algorithms to the data.

The MARC 21 formats provide for using content designation, e.g., tag values or indicators, to specify recommended display constants.  A display constant is a term, phrase, and/or spacing or punctuation convention that may be system generated under prescribed circumstances to make a visual presentation of data in a record more meaningful to a user.  The display constant text is not carried in the data, but may be supplied for display by the processing system.

Variable Fields and Tags:

The data in a MARC 21 record is organized into fields, each identified by a three-character tag.  Although ANSI Z39.2 and ISO 2709 allow both alphabetic and numeric characters, MARC 21 formats use only numeric tags.  The tag is stored in the directory entry for the field, not in the field itself.  Variable field tags are defined in blocks according to the first character of the tag, which, with some exceptions, identifies the general function of the field's data within a record.  The type of information in the field is identified by the remainder of the tag.  The meaning of these blocks depends upon the type of record.

The bibliographic format blocks are:

The authority format blocks are:

The classification format blocks are:

The community information format blocks are:

The holdings format blocks are:

Within some blocks of variable fields, parallels of content designation are preserved, e.g., bibliographic records (1XX, 4XX, 6XX, 7XX, 8XX), authority records (1XX, 4XX, 5XX, 7XX), classification records (70X-75X), and community information records (1XX, 4XX, 6XX, 7XX).  The following meanings are generally given to the final two characters of the tag of fields in these blocks:

Note Fields:

Rules have been developed for the MARC 21 formats that guide when a separate field should be defined for note data and when the data should be included in a general note field.

For the MARC 21 bibliographic format, a specific 5XX note field is defined when at least one of the following is true:

1.  Categorical indexing or retrieval is required on the data defined for the note.  The note is used for structured access purposes but does not have the nature of a controlled access point.

2.  Special manipulation of that specific category of data is a routine requirement.  Such manipulation includes special print or display formatting or selection or suppression from display or printed product.

3.  Specialized structuring of information for reasons other than those given above, e.g., to support particular standards of data content when they cannot be supported in existing fields.

For the MARC 21 authority format, the specifications for notes are covered in the following two conditions:

1.  A specific note field is needed when special manipulation of that specific category of data is a routine requirement.  Such manipulation includes special print or display formatting or selection or suppression from display or printed product.

2.  Multiple notes are generally not established to accommodate the same type of information for different types of authorities.  Notes are thus not differentiated by or limited to subject, name, or series if the same information applies to more than one type.

Local Fields:

Certain tags have been reserved for local implementation.  The MARC 21 formats specify no structure or meaning for local fields.  Communication of such fields between systems is governed by mutual agreements on the content and content designation of the fields communicated.

In general, any tag containing the character 9 is reserved for local implementation within the block structure.  Specifically the 9XX block is reserved for local implementation as indicated above.  The historical development of the MARC 21 formats has left one exception to this general principle:  field 490 (Series Statement) in the bibliographic format.  There are several obsolete fields with tags containing the character 9, (e.g., 009 (Physical Description Fixed-Field for Archival Collection) and 039 (Level of Bibliographic Control and Coding Detail)).

The indicator value 9 and subfield 9 are also reserved for local implementation.

Repeatability:

Theoretically, all fields, except 001 (Control Number) and 005 (Date and Time of Latest Transaction), and subfields may be repeated.  The nature of the data, however, often precludes repetition (e.g., a bibliographic or community information record may contain only one field 245 (Title Statement); an authority or classification record may contain only one 1XX heading field).  The repeatability or nonrepeatability of each field and subfield is specified in the MARC 21 formats.

Coded Data:

In addition to content designation, the MARC 21 formats include specifications for the content of certain data elements, particularly those that provide for the representation of data by coded values.  Coded values consist of fixed-length ASCII character strings.  Individual elements within a coded-data field or subfield are identified by relative character position.  Although coded data occur most frequently in the leader, directory, and variable control fields, any field or subfield may be defined for coded data.

Certain common values for codes used in coded data have been defined:

blank (ASCII 20(hex))  Undefined (element not defined)

n  Not applicable (element not applicable to the item

u  Unknown (record creator was unable to determine value)

z  Other (value other than those defined for the element)

|   Fill character (record creator has chosen not to provide information)

Historical exceptions to these definitions may occur in the formats.  In particular, the blank has been defined as not applicable, or has been assigned a meaning.

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Record Structure