A precision comb is a rake that has stop guides at either end to aid in distributing the colors evenly throughout the bath. This comb is made to use in the 11"x14" tray that comes with the marbling kit. The handle is 18" long with nails spaced 2". 
 
This product is used and very tricky to find - I've decided to sell off the combs I made for teaching my 2.0 classes, get it while you can!