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An International Standard Serial Number (ISSN) is a unique eight-digit number used to identify a print or electronic periodical publication. Periodicals published in both print and electronic form may have two ISSNs, a print ISSN (p-ISSN) and an electronic ISSN (e-ISSN or eISSN). The ISSN system was first drafted as an ISO international standard in 1971 and published as ISO 3297 in 1975. The ISO subcommittee TC 46/SC 9 is responsible for the standard.
The format of the ISSN is an eight digit number, divided by a hyphen into two four-digit numbers. The last digit, which may be 0–9 or an X, is a check digit. The ISSN of the journal Hearing Research, for example, is 0378-5955, the check digit is 5.
To calculate the check digit, the following algorithm may be used:
To confirm the check digit, calculate the sum of all eight digits of the ISSN multiplied by its position in the number, counting from the right (if the check digit is X, then add 10 to the sum). The modulus 11 of the sum must be 0.
ISSN codes are assigned by a network of ISSN National Centres, usually located at national libraries and coordinated by the ISSN International Centre based in Paris. The International Centre is an intergovernmental organization created in 1974 through an agreement between UNESCO and the French government. The International Centre maintains a database of all ISSNs assigned worldwide, the ISDS Register (International Serials Data System) otherwise known as the ISSN Register. As of 2011[update], the ISSN Register contained records for 1,623,566 items.
ISSN and ISBN codes are similar in concept, where ISBNs are assigned to individual books. An ISBN might be assigned for particular issues of a periodical, in addition to the ISSN code for the periodical as a whole. An ISSN, unlike the ISBN code, is an anonymous identifier associated with a periodical title, containing no information as to the publisher or its location. For this reason a new ISSN is assigned to a periodical each time it undergoes a major title change.
Since the ISSN applies to an entire periodical a new identifier, the Serial Item and Contribution Identifier, was built on top of it to allow references to specific volumes, articles, or other identifiable components (like the table of contents).
The ISSN Register is not freely available for interrogation on the web but is available by subscription. There are several routes to the identification and verification of ISSN codes for the general public.
An ISSN can be encoded as a Uniform Resource Name (URN) by prefixing it with "urn:ISSN:". For example Rail could be referred to as "urn:ISSN:1534-0481". URN namespaces are case-sensitive, and the ISSN namespace is all caps. If the checksum digit is "X" then it is always encoded in uppercase in a URN.