015 National Bibliography Number

A National Bibliography is "a source that attempts to list, as comprehensively as possible, the publications of a particular country during a specific period. Publications here can refer to most any kind of intellectual output, regardless of its format." There is an excellent Introduction to National Bibliography created by the University of Illinois Urbana-Champaign's International and Area Studies Library. The definition quoted above is from that article.

The 015 tag contains the entry number for an item found in a national bibliography outside the United States. For example:

To the Webster-Ashburton Treaty: A study in Anglo-American Relations, 1783-1843 by Howard Jones is included in Canadiana, the National Bibliography of Canada. The OCLC record for this title (which is owned by the NHSL if you want to read it) includes:

015 _ _ ǂa 790194139 ǂ2 can

Subfield a contains the entry number for this title.

Subfield 2 tells you which specific National Bibliography the number relates to. To interpret the code refer to the list of National Bibliography Number Source Codes maintained by the Library of Congress. 

You will occasionally see an 015 tag with a subfield q which contains the volume of the National Bibliography that the item is included in. 

As is the case for the 010 tag (where you find the United States' National Bibliography number, more commonly known as an LCCN), in the 015 tag subfield z provides a home for invalid or cancelled National Bibliography numbers.

Any one item might be included in several national bibliographies and thus have multiple numbers assigned to it. Sometimes an item will have more than one entry from a single national bibliography. In either case, the multiple numbers are recorded in separate 015 fields. There are no defined indicators for this tag.