# International Standard Serial Number

An International Standard Serial Number (ISSN) is an eight-digit serial number used to uniquely identify a serial publication. The ISSN is especially helpful in distinguishing between serials with the same title. ISSN are used in ordering, cataloging, interlibrary loans, and other practices in connection with serial literature.

The ISSN system was first drafted as an International Organization for Standardization (ISO) international standard in 1971 and published as ISO 3297 in 1975. ISO subcommittee TC 46/SC 9 is responsible for maintaining the standard.

When a serial with the same content is published in more than one media type, a different ISSN is assigned to each media type. For example, many serials are published both in print and electronic media. The ISSN system refers to these types as print ISSN (p-ISSN) and electronic ISSN (e-ISSN), respectively. Conversely, as defined in ISO 3297:2007, every serial in the ISSN system is also assigned a linking ISSN (ISSN-L), typically the same as the ISSN assigned to the serial in its first published medium, which links together all ISSNs assigned to the serial in every medium.

The format of the ISSN is an eight digit code, divided by a hyphen into two four-digit numbers. As an integer number, it can be represented by the first seven digits. The last code digit, which may be 0-9 or an X, is a check digit. Formally, the general form of the ISSN code (also named "ISSN structure" or "ISSN syntax") can be expressed as follows:

or by a PCRE regular expression:

The ISSN of the journal Hearing Research, for example, is 0378-5955, where the final 5 is the check digit, that is C=5. To calculate the check digit, the following algorithm may be used:

NNNN-NNNC
where N is in the set {0,1,2,...,9}, a digit character, and C is in {0,1,2,...,9,X};
^\d{4}-\d{3}[\dxX]\$.
Calculate the sum of the first seven digits of the ISSN multiplied by its position in the number, counting from the right—that is, 8, 7, 6, 5, 4, 3, and 2, respectively:
${\displaystyle 0\cdot 8+3\cdot 7+7\cdot 6+8\cdot 5+5\cdot 4+9\cdot 3+5\cdot 2}$
${\displaystyle =0+21+42+40+20+27+10}$
${\displaystyle =160}$
The modulus 11 of this sum is then calculated; divide the sum by 11 and determine the remainder:
${\displaystyle {\frac {160}{11}}=14{\mbox{ remainder }}6=14+{\frac {6}{11}}}$
${\displaystyle 0\cdot 8+3\cdot 7+7\cdot 6+8\cdot 5+5\cdot 4+9\cdot 3+5\cdot 2}$
${\displaystyle =0+21+42+40+20+27+10}$
${\displaystyle =160}$
${\displaystyle {\frac {160}{11}}=14{\mbox{ remainder }}6=14+{\frac {6}{11}}}$
If there is no remainder the check digit is 0, otherwise the remainder value is subtracted from 11 to give the check digit:
${\displaystyle 11-6=5}$
5 is the check digit, C.
${\displaystyle 11-6=5}$
For calculations, an upper case X in the check digit position indicates a check digit of 10 (like a Roman ten).
• The print version of a serial typically will include the ISSN code as part of the publication information.
• Most serial 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 serial title.
• WorldCat permits searching its catalog by ISSN, by entering "issn:"+ISSN code in the query field. One can also go directly to an ISSN's record by appending it to "https://www.worldcat.org/ISSN/", e.g. https://www.worldcat.org/ISSN/1021-9749. This does not query the ISSN Register itself, but rather shows whether any Worldcat library holds an item with the given ISSN.
• ISSN is not unique when the concept is "a journal is a set of contents, generally copyrighted content": the same journal (same contents and same copyrights) have two or more ISSN codes. A URN needs to point to "unique content" (a "unique journal" as a "set of contents" reference).
Examples: Nature has an ISSN for print, 0028-0836, and another for the same content on the Web, 1476-4687; only the oldest (0028-0836) is used as a unique identifier. As the ISSN is not unique, the U.S. National Library of Medicine needed to create, prior to 2007, the NLM Unique ID (JID).
• ISSN does not offer resolution mechanisms like a digital object identifier (DOI) or an URN does, so the DOI is used as a URN for articles, with (for historical reasons) no need for an ISSN's existence.
Example: the DOI name "10.1038/nature13777" can be represented as an HTTP string by https://dx.doi.org/10.1038/nature13777, and is redirected (resolved) to the current article's page; but there is no ISSN online service, like http://dx.issn.org/, to resolve the ISSN of the journal (in this sample 1476-4687), that is, a kind of https://dx.issn.org/1476-4687 redirecting to the journal's home.
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