General structure

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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

Líder

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

Directorio

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:

Tipo de registro	Control field
Bibliografía	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

Campos variables

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:

Elemento	Descripción
Líder	A fixed field that occurs at the beginning of each record 24 characters in length
Directorio	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