ISO 2709: The International Standard Exchange Format 

ISO 2709 is an international standard format for bibliographic information interchange on megnatic tape. It was developed for the exchange of bibliographic records on magnetic tape, originally in the Library of Congress MARC format. ISO 2709 structure consists of the following elements:. 

1.  Record label 
2.  Directory 
3.  Data fields 
4.  Record separator 

Record label includes data that identify to the system the type of record and contains information necessary for the processing of the record such as, total number of characters in the record and the length of various elements of the record. 

Directory consists of a content designator for each data field followed by an indication of the position in the record where the data relating to that field start and the length of the field. If a field is repeated, it has two entries in the directory, one for each appearance. 

Datafield is a variable-length portion of the particular category of data associated with one entry in the directory. A datafield may contain one or more subfield. 

Record separator is the final character of each record. This will always be character 1/13 of ISO 646.