The flat keystone is crafted in urethane and designed for use with 12" and 13" crossheads. The back of the keystone interlocks with the moulding profile for a perfect fit. When enhanced with a keystone, the crosshead brings standout detail to entryways and windows in both interior and exterior applications. The keystone adds detail to arches, crossheads and headers, mouldings and more. The decorative keystone is easy to install for truly impressive results, whether you require something subtle or bold in appearance. The flat keystone is molded with precision in high quality polyurethane materials. High-density polyurethane is also lightweight, non-porous, pre-primed and easy to work with.