a horizontal crossbar in a window, over a door, or between a door and a window above it. transom is the horizontal, as mullion is the vertical, bar across an opening. see illust. of mullion.-one of the principal transverse timbers of the stern, bolted to the sternpost and giving shape to the stern structure; -- called also transsummer.-the piece of wood or iron connecting the cheeks of some gun carriages.-the vane of a cross-staff.-one of the crossbeams connecting the side frames of a truck with each other.