In article <884 at amethyst.ma.arizona.edu> rsm at amethyst.ma.arizona.edu (Robert Maier) writes: [ ISBN's are in the form ...] >with a weighted mod 11 checksum. (The mysterious "X" in the checksum >field, which one occasionally encounters, means 11.) I've seen this twice. Surely not. In arithmetic mod 11 one needs a new symbol for 10, not 11. Which is what the X is for. D