Keystones are designed to go above doors and windows, to add flair and sophistication. Polyurethane keystones are light weight compared to precast concrete keystones, and are can help meet budget constrains due to a lower manufacturing cost. To maximize your budget, consider using the more ornate keystones around entry doors, garage doors and large windows, and smaller or plain keystones elsewhere. Keystones are simple to install due to their light weight, and just require adhesive to be attached to a wall.