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  (character positions 00-02)  Contains 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 zeros (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 zeros (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 for order requirements for the fields to which the directory entries point.

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Record Structure