Why Do Some ISSN End in an “X”?
An ISSN is composed of eight digits: the arabic numerals 0 to 9, except that in the case of the last digit, which is a check digit, the upper case X can appear. The purpose of the check digit is to guard against errors caused by the incorrect transcription of the ISSN. The method of determining the check digit for the ISSN is the modulus 11 basis, using the weighting factors 8 to 2. In the case of the ISSN, the Roman numeral X is used where the check digit calculation results in a remainder of 10.
