Disclaimer: This resource is meant to be a helpful way for cataloging students and new catalogers to understand and use the most basic elements of MARC 21.
For official descriptions of rules and use, please refer to the Library of Congress MARC 21 website.

This site is intended to be an aid for those being introduced to the MARC 21 format, or beginning to use it. It will discuss the most common fields and indicators used, as well as offer advice on when and how to use them. Please note that I myself am a fairly novice cataloger, and this is only meant to provide some help for MARC's most basic aspects.

All examples follow the guidelines set forth by the Anglo-American Cataloging Rules, 2nd edition, revised.

What is MARC 21?
The MARC 21 (MAchine Readable Cataloging) format is, simply put, the international standard for creating computerized bibliographic records. These records can be shared amongst other libraries online, usually via a shared cataloging network like OCLC. MARC encodes the various descriptive elements of a resource - title, author, physical elements, subjects, etc., into specified fields, each with numerical indicators, that the program recognizes and translates into the data seen onscreen.

What is a field?
MARC fields are, for the most part, each individual component of a bibliographic record. Each 3-digit field is dedicated to a certain aspect of a resource's intellectual and physical properties, like title or publisher. Most fields are comprised of at least one subfield, which further details the contents of a field; they are represented alphabetically, with each letter having a different meaning with each field.

Example: 245 (this is a basic title field featuring the info seen on the title page)
$a (subfield (represented by "$") "a", the main title) Ernest Hemingway
$b (subfield "b", any subtitle or additional part of a title) a life story
$c (subfield "c", the author or "statement of responsibility") By Carlos Baker.
Altogether, it should look like this:
245 $a Ernest Hemingway : $b a life story / $c By Carlos Baker.

What is an indicator?
Indicators are numbers that appear after a field "tag" (the three-digit number) to help further describe to the computer what's in a field. Up to two indicators can be used, and all are field-specific. Some fields only use one indicator, other don't use any at all.

Example: The 245 (title) field uses two indicators:
Indicator 1 - Title added entry: 1=added entry needed (the title would require a separate title tracing), 0=no added entry needed
Indicator 2 - Nonfilling characters: how many spaces due to common articles (such as "the" or "a") the program should ignore before getting to the title's first valid word, including the space after the article
If we use the above example, the indicators would be: "245 00".
The title is not traced separately, and no spaces are needed before first valid word in title, so indicator 2 is 0.

In its totality, the field used in our example should look like this:
245 00 $a Ernest Hemingway : $b a life story / $c By Carlos Baker.