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General structure

Find an overview of the general structure, including control fields and variable fields, of OCLC MARC records.

In MARC 21 formats, the record structure is the order in which the content designators and content appear in the record and/or file. Record structure can include such specifications as tape media, header, blocking techniques, and characters sets used in the record and in files.

  • Note that the terms record format and the record structure are often used interchangeably
  • Record format is generally the broader term and often is used to refer to the record structure, content designation, and content of the record


The leader consists of data elements that contain coded values and are identified by relative character position

  • Data elements define parameters for processing the record
  • Length = 24 characters


The record directory consists of a series of fixed-length entries containing the tag, length, and starting character position of each control and variable field

  • Length = 12 characters per entry
  • 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 digit of the tag

Control fields

Control fields are assigned tags that begin with 2 zeros

  • They contain data and a field terminator
  • They do not contain indicators or subfield codes
  • They include data useful for record selection or special processing

The following are currently implemented control fields listed by record type:

Record type Control field
Bibliographic 001, 003, 005, 006, 007, 008
Authority 001, 003, 005, 008

003 control field

Depending on the service from which the record is received, it has one of the following in the 003 control field:

  • In exported records, the 003 field contains OCLC’s organization symbol OCoLC
  • In original records from the OCLC CatCD® for Windows system, the 003 field contains the user-input control number identifier

Variable fields

  • Variable data fields may differ in content for each type of MARC record
  • They consist of variable-length alphanumeric data
  • They begin with 2 indicators
  • Subfield delimiters and codes separate alphanumeric data within variable fields
  • No spaces appear before or after subfield delimiters and codes
  • All fields end with a field terminator
  • The record terminator follows the last field in the record

Elements of general record structure

Each bibliographic or authority record has the following elements:

Element Description
  • A fixed field that occurs at the beginning of each record
  • 24 characters in length
  • An entry that contains the tag, length, and starting position for each fixed and variable field in the record
  • A field terminator ends the record directory
  • 12 characters in each directory entry
Control Field
  • A variable field that contains information useful or required for processing
  • Assigned tags begin with two zeros
  • Contains alphanumeric data
  • Positions within the control field identify specific data elements
Data Field
  • A variable field containing bibliographic or other data
  • Assigned tags do not begin with two zeros
  • Not necessarily in numeric sequence within a block of tags
  • The last field ends with a field terminator and a record terminator
Field Terminator
  • ASCII character which ends the directory and each variable field within a record
  • Also known as a record separator
Record Terminator
  • ASCII character that is the final character of a record, following the field terminator of the last data field
  • Also known as a group separator
  • Before December 8, 1984, the last field ended with a record terminator; the separate field terminator was not used