Following the Leader

In the first post of this series I included this definition:

Leader. The part of a MARC record that contains 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.

Over the next several weeks I will be discussing various pieces of the leader and how they function. 

All MARC bibliographic records have a leader. If you use OCLC's cataloging interface you may be familiar with the term "fixed field." The OCLC fixed field includes the 24 bytes of the leader as well as some other stuff and presents it (by default) as a labelled set of data.

There are 24 character positions (referred to as bytes) in the leader and they are identified by numbers from 00 to 23. 

If you are not a computer then there are 17 out of the 24 bytes that you won't need to know anything about and should not mess with. 

 For those of you who like to know extra stuff, this is what is in those positions:

An example of a leader with the bytes I am about to describe displayed in different colors:

02595cam_a2200469_a_4500

I will be using this same leader as an example in future posts and will tell you later on what record it came from.

00-04 Logical Record Length: a five-character number (with leading zeros for shorter numbers) equal to the length of the entire record, including these 5 digits and the record terminator. (Red in the example)

09 Character Coding Scheme: Identifies the character coding scheme used in the record. MARC-8 is indicated by a blank and Unicode is indicated by "a". (Aqua in the example)

10 Indicator Count: This is always 2 and indicates the number of character positions used for indicators in a variable data field. (Tan in the example)

11 Subfield Code Count: This is always 2 and indicates the number of character positions used for each subfield code in a variable data field. (Bright green in the example)

12-16 Base Address of Data: a five-character number (with leading zeros for shorter numbers) that indicates the first character position of the first variable control field in a record. (Fuchsia in the example)

These positions as a group are the "Entry Map" (blue in the example) and tell the computer things about the Directory portion of the record:

20 Length of the Length-of-Field Portion: This is always 4 and is the number of characters in the length-of-field portion of a Directory entry

21 Length of the Starting-Character-Position Portion: This is always 5 and is the number of characters in the starting-character-position portion of a Directory entry

22 Length of the implementation-defined portion: This is always 0 and is the number of characters in the implementation-defined portion of a Directory entry. Note that OCLC uses this byte for the Transaction Type Code in Hexadecimal. 

23 Undefined: This is always 0.