copied this from another forum:
"International Standard ISO 5775-2:1996, Bicycle tyres and rims — Part 2: Rims: document:
"ISO 5775-2 defines designations for bicycle rims. It distinguishes between
Straight-side (SS) rims
Crotchet-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."
some examples would be: SS 400×20, HB 422, 700x23c, the "C" designating a Crotchet rim.