ISSN - International Standard Serial Number

International Standard Serial Number
The International Standard Serial Number (ISSN) is an international code for serial publications like magazines.
The ISSN consists of two groups of four digits, separated by a hyphen. The eighth character is a check digit calculated according to a modulo 11 algorithm on the basis of the 7 preceding digits. This eighth control character may be an "X" if the result of the calculation is equal to "10".

To calculate the check digit you must multiply the last digit of the true number by 2, the second to the last by 3 etc. and add these results. The number needed to fill this sum to the next multiple of 11 is the check digit. If it is 10, the check digit is replaced by the letter 'X'.

More detailed information about the structure of ISSN numbering is available here.

Bar Code and ISSN
The barcode symbols that appear on magazines are encoded using EAN-13 barcode. However, although the type of bar code is the same as others used for retailers, the numbering system used to generate the bar code is different. The EAN for normal retail products is a 13 digit number which uniquely identifies that product. However, a serial publication like a magazine already has a unique number to identify it, the ISSN. The EAN bar code for a serial publication is generated from the ISSN for the publication.

When encoded in an EAN-13 bar code, the ISSN is preceded by the digits 977 and the ISSN check-digit (the last of the 8 digits) is not encoded. A two digit price code, almost always "00", is added to the end, and finially the EAN check digit is added.

Let's say you have the number 0264-3596 as a serial publication's ISSN. The true ISSN is 0264359 (no 6 because it is the ISSN check digit). The EAN-13 number would then be 9770264359008 for normal issues. The "00" is the normal issue code and the "8" is the EAN-13 check digit). A special issue could receive a 2-digit numbers other than "00".

Some serial publications also have an add-on 2-digit code that represents the issue number.

You can find much more information about how to apply for an ISSN code and other information at ISSN.ORG.

There is more information about ISSN at the United States Library of Congress.