These guide rails are designed to help maintain the correct angle for sharpening knives when sharpening with a whetstone. The small size Guide Rail should be used for knives with blade width between 15mm and 25mm. Large size Guide Rail should be used for knives with blade width between 27mm and 43mm. Any time you remove one of the guide rails from your knife, rinse and clean it thoroughly to remove any residue from the stone that may remain inside the guide rail. To further insure that no residue remains, it is recommended that you pull a clean towel through the guide rail after rinsing. If you fail to remove all the residue from the guide rail, the residue may scratch the knife the next time the guide rail is used.