The check digit in the ISSN is the righmost (eighth) digit.  It is, I believe,
called a "modulus 10" check digit.  This scheme can sometimes result in a
check digit of "10."  Since the ISSN is restricted by definition to eight
digits, the Roman numeral "X" is used whenever a check digit of ten is
calculated.  If there is further interest, perhaps someone with a copy of the
ANSI standard handy could post the actual method of calculation.
B.J. Segel
Pepperdine Law Library