I know this is an old thread, but since it's currently the first hit that comes up in The Google:
From The Wikipedia:
ISO 5775-2 defines designations for bicycle rims. It distinguishes between
- Straight-side (SS) rims
- Crochet-type (C) rims
- Hooked-bead (HB) rims
Rims are designated by their nominal rim diameter and their nominal width, separated by a cross (×). Both are measured in millimeters. The rim type codes SS or HB precede the rim designation, whereas code C is appended to the nominal width.
Examples:
SS 400×20, HB 422×25, 620×13C
The nominal width of a rim is the inner width between the straight sides or beads as one can easily measure it with a caliper (see the standard for drawings and exact measurement procedures).
The standard widths of straight-side rims are:
18, 20, 22, 24, 27, 30.5
The standard widths of crochet-type rims are:
13C, 15C, 16C, 17C, 19C, 21C, 23C, 25C
http://en.wikipedia.org/wiki/ISO_5775