Box cornices enclose the cornice of the building with what is essentially a long narrow box. A box cornice may further be divided into either the "narrow" box cornice or the "wide" box cornice type. A narrow box cornice is one in which "the projection of the rafter serves as a nailing surface for the soffit board as well as the fascia trim." This is possible if the slope of the roof is fairly steep and the width of the eave relatively narrow. A wide box cornice, which is common practice on houses with gentle roof slopes and wide eaves, requires the use of lookouts to give it support and to provide a surface to which to securely attach the soffits. Box cornices often have ventilation screens laid over openings cut in the soffits in order to allow air to circulate within the cornice.