MARC Communications Format
This is a picture of a "MARC dump." It shows the leader and the directory followed by the control fields, fixed fields, and variable fields of the record. The first information in the leader is that the record is 723 characters long. This information is followed by characters in fixed positions that give specific coded information.
The directory is divided into segments of 12 characters each. There are as many 12-character segments as there are fields in the rest of the record. The first three characters of each 12-character segment are the MARC tag for the field represented. The next 4 characters tell the length (number of characters) of the field represented. The last five characters tell the starting position for that field in the portion of the record that follows the directory. For example, the first segment of the directory above tells that the 001 field has 13 characters and starts at position 0 in the record immediately after the directory; the second segment tells that the 008 field has 41 characters and starts at position 13; the third segment tells that the 005 field has 17 characters and starts at position 54; and so on.
The directory allows a programmer to write a program for display that essentially says to the computer: go to position 0 [or 13, or 54, etc.], get the next 13 [or 41, or 17, etc.] characters, go to the display and place those characters at position x, label this field "LCCN" [or "fixed data," or "date/time," etc.].