ISSN (International Standard Serial Number) structure:

The ISSN is an agency-assigned data element.  ISSNs are assigned to continuing resource publications by national centers under the auspices of the ISSN Network.

An ISSN consists of eight digits comprising two groups of four digits each, separated by a hyphen.  The eighth digit is a check digit used as a computer validity check; it consists of a number between 0 and 9 or an uppercase X (for the arabic numeral 10).

A description of the ISSN structure and the procedure for validation of the ISSN by calculating the check digit is in International Standard Serial Numbering (ISSN) (ISO 3297).