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  (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 zeros (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.

Bibliographic level  (bibliographic record, character position 07)  Contains an ASCII graphic character which also provides information about the components and characteristics of the record.

Kind of data  (community information record, character position 07)  Contains an ASCII graphic character which also provides information about the components and characteristics of the record.

Type of control  (bibliographic record, character position 08)  Contains an ASCII graphic character which also provides information about the components and characteristics of the record.

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 zeros (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 if 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.

Length of length-of-field  (character position 20)  Specifies the length of that part of each directory entry; in MARC 21 records, it is always set to 4.

Length of starting-character-position  (character position 21)  Specifies the length of that part of each directory entry; in MARC 21 records, it is always set to 5.

Length of implementation-defined  (character position 22)  Specifies that part of each directory entry; in MARC 21 records, a directory entry does not contain an implementation-defined portion, therefore this position is always set to 0.

Undefined  (character position 23)  This character position is undefined; it is always set to 0.

Structure of an Entry Map in MARC 21 Records

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Record Structure