Even when an ISSN is used in a bar-code it should also be printed in full on the publication. This, main, part of the bar-code identifies the title. This is generated from a 2-digit number. The first sequence variant is used, as above, to indicate a price change. The check-digit will also change.
Some guidelines are also available for extra or special issues and for temporary price changes.
Annual publications, or publications which appear less frequently, may sometimes have either an ISSN or an ISBN or both. An ISBN is a ten-digit number, consisting of a prefix, which indicates the Centre that assigned it, followed by the digits assigned by the Centre to identify the publisher, which are in turn followed by the digit or digits which identify the specific item. The final digit is a check digit, calculated by a computer programme on the basis of the preceding digits, to ensure the number has been correctly transcribed.