ISSN (International Standard Serial Number) is an identifier for periodical publications such as magazines and journals, both print and electronic. It has been standardized as ISO 3297.
ISSNs have 8 digits, conventionally printed in two groups of four digits separated by a hyphen. The final digit is a checksum. ISSNs are assigned free of charge by national organizations (in the United States this is done by the Library of Congress).
When a publication changes its title, it is expected to apply for a new ISSN for the newly-named publication, since ISSNs are keyed to the particular title. The central database of ISSNs has cross-references reflecting past titles and ISSNs of a publication. Sometimes separate ISSNs are issued for the print and online versions of a publication.
There is a coding scheme to allow ISSNs to be represented on EAN-13 bar codes for retail sale, with some prefixes and suffixes added to indicate publication type and format, and a different checksum digit. This is usually accompanied by an EAN-2 or EAN-5 supplemental code designating the particular issue.
Some serials (such as periodically-issued reference books) also use ISBNs for the individual volumes of the series since they are sold separately as books as well as by subscription, so they have both ISBNs and ISSNs.