Contains a series of entries that contain the tag, length, and starting location of each variable field within a record. Each entry is 12 character positions in length. Directory entries for variable control fields appear first, sequenced by tag in increasing numerical order. Entries for variable data fields follow, arranged in ascending order according to the first character of the tag. The stored sequence of the variable data fields in a record does not necessarily correspond to the order of the corresponding Directory entries. Duplicate tags are distinguished only by the location of the respective fields within the record. The Directory ends with a field terminator character (ASCII 1E hex).

The data in a MARC 21 record is organized into variable fields, each identified by a three-character numeric tag that is stored in the Directory entry for the field. Each field ends with a field terminator character. The MARC record ends with a record terminator (ASCII 1D hex). There are two types of variable fields:

The tagged 00X fields. The variable control fields are structurally different from the variable data fields

. These fields contain neither indicator positions nor subfield codes. They may contain either a single data element or a series of fixed-length data elements identified by relative character position.