is the "ridge" on the top of the koi that extend from the front of the dorsal to the head. Many GRs do not have GR scalation along this line and seem bare.
Take a look at the GR Kohakus:
Maruju Tategoi Tosai From Genki - KoiShack
The best example of one having GR scalation onm this ridgeline is the one on the lower right. Based on the pictures, none of the others do (but pictures can most definitely lie).
Make sense now?
Steve