The keystone lends support to the arch in structural applications. The decorative appeal of the keystone continues to endure, and the decorative keystone proves versatile in countless applications. The flat keystone is designed for use with 10" Decorative Arch Trim. The flat keystone is molded to interlock with the decorative arch profile. The keystone is molded with precision for a perfect fit. It is crafted in rigid, high-density polyurethane. As with our urethane trim products, the flat keystone performs exceptionally well in exterior applications, so whether you are enriching an interior bedroom window or enhancing an exterior entryway, this product remains free of peeling, cracking and decay.