International Standard Serial Number

# Overview
An International Standard Serial Number (ISSN) is a unique eight-digit number used to identify a print or electronic periodical publication. The ISSN system was adopted as international standard ISO 3297 in 1975. The TC 46/SC 9 is responsible for the standard.

# Code format
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 09 or an X, is a check digit.
The ISSN of the journal Hearing Research, for example, is 0378-5955. To confirm the check digit, the following algorithm may be used:
Calculate the sum of the first seven digits of above ISSN multiplied by 8, 7, 6, 5, 4, 3, and 2, respectively:
The modulus 11 of this sum is then calculated. Some calculators have a mod() function:
Alternatively you can divide the sum by 11 and determine the remainder:
This modulus or remainder value is then subtracted from 11 to give the check digit:
An upper case X in the check digit position indicates a check digit of 10.

# Code assignment
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. The ISSN Register contains ISSN codes and descriptions for more than one million periodicals with around 50,000 new records added yearly.

# Comparison to other identifiers
ISSN and ISBN codes are similar in concept, where ISBNs are assigned to individual books.  For particular issues of a periodical an ISBN might be assigned in addition to the ISSN code for the periodical as a whole. Unlike the ISBN code, an ISSN 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 referencing specific volumes, articles, or other identifiable components (like the table of contents).

# Availability
The ISSN Register is not freely available for interrogation on the web but is available on a subscription basis.  There are several routes to the identification and verification of ISSN codes for the general public.
- the print version of a periodical typically will include the ISSN code as part of the publication information
- most, though not all, periodical websites contain ISSN code information
- derivative lists of publications will often contain ISSN codes; these can be found through on-line searches with the ISSN code itself or periodical title

# Use in URNs
An ISSN can be encoded as a URN by prefixing it with "urn:issn:".  For example Rail could be referred to as "urn:issn:1534-0481".  If the checksum digit is "X" then it is always encoded in uppercase in a URN.