We offer keystone ranging from detailed to basic, and the flat keystone is a versatile example. It is designed for use with 6" and 7" crossheads with bottom trim. The back of the keystone interlocks perfectly with the crosshead's moulding profile. Accentuate trim with the addition of a decorative keystone. It installs easily for beautiful results. The flat keystone is part of our extensive urethane trim selection, which offers affordability and high performance in both interior and exterior applications. The flat keystone is non-porous. Low moisture absorption prevents peeling, cracking and decay for years of use with little maintenance required. The keystone is also pre-finished in an exterior grade primer.