Components of a MARC Record

A MARC record consists of three main components: the Leader, the Directory, and the Variable Fields. The following information summarizes the structure of a MARC record. More detail is provided in MARC 21 Specifications for Record Structure, Character Sets, and Exchange Media.

Description of Record Parts:

1. Leader - Data elements that primarily provide information for the processing of the record. The data elements contain numbers or coded values and are identified by relative character position. The Leader is fixed in length at 24 character positions and is the first field of a MARC record. (See: Leader.)

2. Directory - 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). (See: Directory.)

3. Variable Fields - The data in a MARC 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 last variable field in a record ends with both a field terminator and a record terminator (ASCII 1D hex). There are two types of variable fields:

Variable control fields - The 00X fields. These fields are identified by a field tag in the Directory but they contain neither indicator positions nor subfield codes. The variable control fields are structurally different from the variable data fields. They may contain either a single data element or a series of fixed-length data elements identified by relative character position.

Variable data fields - The remaining variable fields defined in the format (0XX-9XX). In addition to being identified by a field tag in the Directory, variable data fields contain two indicator positions stored at the beginning of each field and a two-character subfield code preceding each data element within the field. (See: Content Designators for additional information on indicator positions and subfield codes.)

The variable data fields are grouped into blocks according to the first character of the tag, which with some exceptions identifies the function of the data within the record. The type of information in the field is identified by the remainder of the tag. For example, in all five formats, the 0XX block contains Number and Code fields for control information, identification and classification numbers, etc.

Within blocks in some of the formats, certain parallels of content designation are preserved. Meanings, with some exceptions, are given to the final two characters of the tag of fields. In the bibliographic format, they are:

X00 Personal names	X40 Bibliographic titles
X10 Corporate names	X50 Topical terms
X11 Meeting names	X51 Geographic names
X30 Uniform titles