MARC 21 is an implementation of the American national standard, Information Interchange Format (ANSI Z39.2) and its international counterpart, Format for Information Exchange (ISO 2709). These standards specify the requirements for a generalized interchange format that will accommodate data describing all forms of materials susceptible to bibliographic description, as well as related information such as authority, classification, community information, and holdings data. The standards present a generalized structure for records, but do not specify the content of the record and do not, in general, assign meaning to tags, indicators, or data element identifiers. Specification of these elements are provided by particular implementations of the standards. The following description of the MARC 21 record structure indicates the specific choices made for the MARC 21 implementation of the standards.
Italicized terms within definitions are terms for which definitions are also provided.
or #), which is used in indicators and data elements containing coded
values (and occurs in data content). Generally, blank stands for "undefined,"
but in some instances it has been assigned a meaning. ASCII name is space.
or $), which is combined with a data element identifier to make up
the subfield code which precedes each individual data element
within a variable field. The ASCII name for the delimiter is unit
separator (US).MARC 21 records are character encoded, including all lengths. In this section on Record Structure, elements may be specified as ASCII numeric characters, ASCII lowercase alphabetic characters, ASCII uppercase alphabetic characters, ASCII graphic symbol characters, ASCII control characters, ASCII blank character, ASCII graphic characters, or MARC 21 characters. The section on Character Sets defines the repertoire and encoding of each of these subsets of characters.
The general structure of a record is represented schematically below.
Structure of a MARC 21 Record
LEADER DIRECTORY FT CONTROL_NUMBER_FIELD FT
CONTROL_FIELD_1 FT ... CONTROL_FIELD_n FT
DATA_FIELD_1 FT ... DATA_FIELD_n FT RT
Each record begins with a leader, which is a fixed field containing information for the processing of the record. Following the leader is the directory, which is an index to the location of the variable fields (control and data) within the record. The fields following the directory are all variable fields. The first variable field is the control number field, which contains an ASCII graphic character string uniquely associated with the record by the organization transmitting the record. Following the control number field are the rest of the control fields, which contain information useful or required for the processing of the record. Following the control fields are data fields, which contain general data. A field terminator (FT), ASCII control character 1E(hex), is used to terminate the directory and each variable field within the record. A record terminator (RT), ASCII control character 1D(hex), is used as the final character of the record, following the field terminator of the last data field. These elements of the record are described in more detail in the following sections.
The leader is the first field in the record and has a fixed length of 24 octets (character positions 0-23). Only ASCII graphic characters are allowed in the Leader. The structure of the leader as defined in MARC 21 is represented schematically below. The numbers indicate the character positions occupied by each part of the leader.
Structure of the Leader in MARC 21 Records
RECORD_LENGTH RECORD_STATUS TYPE_OF_RECORD IMPLEMENTATION-
DEFINED
00-04 05 06 07-08
CHARACTER_CODING_SCHEME INDICATOR_COUNT SUBFIELD_CODE_LENGTH
09 10 11
BASE_ADDRESS_OF_DATA IMPLEMENTATION-DEFINED ENTRY_MAP
12-16 17-19 20-23
Record length (character positions 00-04), contains a five-character ASCII numeric string equal to the length of the entire record, including itself and the record terminator. The five-character numeric string is right justified and unused positions contain zeroes (zero fill). The maximum length of a record is 99999 octets.
Record status (character position 05), contains an ASCII graphic character which indicates the relation of the record to a file (e.g., new, updated, etc.).
Type of record (character position 06), contains an ASCII graphic character which specifies the characteristics and defines the components of the record.
Implementation-defined (character positions 07-08). ANSI Z39.2 and ISO 2709 reserve character positions 07-08 for definition by a particular implementation. The individual MARC 21 formats define these character positions if needed. Positions may contain only ASCII graphic characters. Any position not defined contains a blank.
Character coding scheme (character position 09), contains a code that identifies the character coding scheme used in a record.
Indicator count (character position 10), contains one ASCII numeric character specifying the number of indicators occurring in each variable data field. In MARC 21 records, the indicator count is always 2.
Subfield code length (character position 11), contains one ASCII numeric character specifying the sum of the lengths of the delimiter and the data element identifier used in the record. In MARC 21 records, the subfield code length is always 2. The ANSI Z39.2 and ISO 2709 name for this data element is identifier length .
Base address of data (character positions 12-16), contains five ASCII numeric characters that specify the first character position of the first variable field in the record. It is equal to the sum of the lengths of the leader and the directory, including the field terminator at the end of the directory. The number is right justified and unused positions contain zeroes (zero fill).
Implementation-defined (character positions 17-19). ANSI Z39.2 and ISO 2709 reserve character positions 17-19 for definition by a particular implementation. The individual MARC 21 formats define these character positions is needed. Positions may contain only ASCII graphic characters. Any position not defined contains a blank.
Entry map (character positions 20-23), contains four single digit ASCII numeric characters that specify the structure of the entries in the directory.
Structure of an Entry Map in MARC 21 Record
LENGTH OF LENGTH OF LENGTH OF LENGTH-OF-FIELD STARTING-CHARACTER- IMPLEMENTATION- PART POSITION PART DEFINED PART UNDEFINED 20 21 22 23
A directory entry in MARC 21 is made up of a tag, length-of-field, and field starting position. The directory begins in character position 24 of the record and ends with a field terminator. It is of variable length and consists of a series of fixed fields, referred to as "entries." One entry is associated with each variable field (control or data) present in the record. Each directory entry is 12 characters in length; the structure of each entry as defined in MARC 21 is represented schematically below. The numbers indicate the character positions occupied by the parts of the entry.
Structure of a Directory Entry in MARC 21 Records
TAG LENGTH_OF_FIELD STARTING_CHARACTER_POSITION
00-02 03-06 07-11
Tag (character positions 00-02), consists of three ASCII numeric characters or ASCII alphabetic characters (uppercase or lowercase, but not both) used to identify or label an associated variable field. The MARC 21 formats have used only numeric tags. The tag is stored only in the directory entry for the field; it does not appear in the variable field itself.
Length of field (character positions 03-06), contains four ASCII numeric characters which give the length, expressed as a decimal number, of the variable field to which the entry corresponds. This length includes the indicators, subfield codes, data and field terminator associated with the field. A field length number of fewer than four digits is right justified and unused positions contain zeroes (zero fill). MARC 21 sets the length of the length of field portion of the entry at four characters, thus a field may contain a maximum of 9999 octets.
Starting character position (character positions 07-11), contains five ASCII numeric characters which give the starting character position, expressed as a decimal number, of the variable field to which the entry corresponds relative to the base address of data of the record. A starting character position of fewer than five digits is right justified and unused positions contain zeroes (zero fill).
Order of entries Directory entries for control fields precede entries for data fields. Entries for control fields are sequenced by tag in increasing numerical order. Entries for data fields are arranged in ascending order according to the first character of the tag, with numeric characters preceding alphabetic characters. See Variable Fields below for order requirements for the fields to which the directory entries point.
The variable fields follow the leader and the directory in the record and consist of control fields and data fields. Control fields precede data fields in the record and are arranged in the same sequence as the corresponding entries in the directory. The sequence in which data fields are stored in the record is not necessarily the same as the order of the corresponding directory entries.
Control fields in MARC 21 formats are assigned tags beginning with two zeroes. They are comprised of data and a field terminator; they do not contain indicators or subfield codes. The control number field is assigned tag 001 and contains the control number of the record. Each record contains only one control number field (with tag 001), which is to be located at the base address of data.
Data fields in MARC 21 formats are assigned tags beginning with ASCII numeric characters other than two zeroes. Such fields contain indicators and subfield codes, as well as data and a field terminator. There are no restrictions on the number, length, or content of data fields other than those already stated or implied, e.g., those resulting from the limitation of total record length. The structure of a data field is shown schematically below.
Structure of a Variable Data Field in MARC 21 Records
INDICATOR_1 INDICATOR_2 DELIMITER DATA_ELEMENT_IDENTIFIER_1
DATA_ELEMENT_1 ... DELIMITER DATA_ELEMENT_IDENTIFIER_n
DATA_ELEMENT_n FT
Indicators are the first two characters in every variable data field, preceding any subfield code (delimiter plus data element identifier) which may be present. Each indicator is one character and every data field in the record includes two indicators, even if values have not been defined for the indicators in a particular field. Indicators supply additional information about the field, and are defined individually for each field. Indicator values are interpreted independently; meaning is not ascribed to the two indicators taken together. Indicators may be any ASCII lowercase alphabetic, numeric, or blank. A blank is used in an undefined indicator position, and may also have a defined meaning in a defined indicator position. The numeric character 9 is reserved for local definition as an indicator.
Subfield codes identify the individual data elements within the field, and precede the data elements they identify. Each data field contains at least one subfield code. The subfield code consists of a delimiter (ASCII 1F (hex)) followed by a data element identifier. Data element identifiers defined in MARC 21 may be any ASCII lowercase alphabetic or numeric character. In general, numeric identifiers are defined for data used to process the field, or coded data needed to interpret the field. Alphabetic identifiers are defined for the separate elements which constitute the data content of the field. The character 9 and the following ASCII graphic symbols are reserved for local definition as data element identifiers:
! " # $ % & ' ( ) * + , - . / : ; < = > ? { } _ ^ ` ~ [ ] \
A data field may contain more than one data element, depending upon the definition of the field. The last character in a data field is the field terminator, which follows the last data element in the field.
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 above but is given again for completeness.
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.
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:
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:
For the MARC 21 authority format, the specifications for notes are covered in the following two conditions:
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.
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.
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:
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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