Excerpted from the ISSN Manual, a publication of the ISSN International Network
The purpose of a check digit is to guard against errors caused by the incorrect transcription of an ISSN. The modulus 11 basis using the weighting factors 8 to 2 for calculating the check digit is one of the most efficient systems for detecting transcription errors.
The procedure for calculating the check digit, which may be carried out automatically in a computer, is as follows:
If the remainder is 10, substitute an upper case X in the check digit position. If there is no remainder, put a zero in the check digit position.
It should be noted that the check digit is an essential and inseparable part of the ISSN.
Library of Congress Help Desk ( February 19, 2010 )