here's wikis take on it
http://en.wikipedia.org/wiki/ISO_5775
and:
For wired-edge tires the ISO designation lists the width of the inflated tire and the diameter with which the tire sits on the rim, both in millimeters and separated by a hyphen: 37-622
For beaded-edge tires the ISO designation lists an overall diameter code (16, 18, 20, 22, 24, or 26) and a width code (1.25, 1.375, 1.75, or 2.125), defined by measurement tables given in the standard, separated by a cross: 20×1.375
For rims the ISO designation lists the rim diameter (where the tire sits) and the rim's inner width, both in millimeters and separated by a cross, along with a letter code for the rim type (e.g., "C" = Crotchet-type): 622x19C
source:
http://en.wikipedia.org/wiki/Bicycle_wheel
Last edited by imi; 12-22-10 at 02:38 PM.