Many of the Keystones on this page are intended to be used with specific Poly Mouldings. This is noted with each Keystone. Example: "For 5-1/2" Flat Board".
Other Keystones on this page have flat backs and can be used with any Moulding that will butt against the side of the Keystone without projecting beyond the face of the Keystone. Read more about Polyurethane.