Marc 21 Record Format -- Directory Section

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MARC21 ::= Leader + Record Directory + Variable Fields + X'1B'

Directory ::= Tag + Field Length + Position + Directory -- char(12)

A computer-generated index to the location of the variable control and data fields within a record. The Directory immediately follows the Leader at character position 24 and consists of a series of fixed-length (12 character positions) entries that give the tag, length, and starting character position of each variable field.

Tag ::= 'AAA' -- char(3)

Three numeric or alphabetic (uppercase or lowercase, but not both) characters that identify an associated field.

Field Length ::= '9999' -- char(4)

Four numeric characters that indicate the length of the field, including indicators, subfield codes, data, and the field terminator. The number is right justified and each unused position contains a zero.

Position ::= '99999' -- char(5)

Five numeric characters that indicate the starting character position of the field relative to the Base address of data (Leader/12-16) of the record. The number is right justified and each unused position contains a zero.