Abstract: A mate line closure and aesthetic structural embellishment for structures, included modular buildings. A pair of elongated modular elements, forming part of a modular unit, are provided, having junction ends proximate one another when assembled as the modular unit. A key joins the modular elements at the junction ends, the key forming a finished union with opposite side plates and a bottom plate. The modular unit is installed in portions in a building, such as at a mate line, and is preferably secured to an under support installed before the elements of the modular unit are then installed.