The ISSN identifies ”serial publications” – journals, magazines, annual conference proceedings. Developed on the heels of the ISBN, the ISSN was drafted in 1971 and published by ISO in 1975. It was clear from the ISBN’s success that numbering publications was a good idea.
The ISSN, unlike the ISBN, is a dumb number. The digits carry no intrinsic meaning; there are no prefixes or groups. The ISSN is an eight-digit number – seven digits and a checksum.
Also unlike the ISBN, there is such a thing as an e-ISSN.
As you might expect, it identifies the digital version of the journal. There is, therefore, a p-ISSN, identifying the print version. Linking the two is the ISSN-L, which is assigned to whichever version is published first.