The fourth "spine" side has flexible joints where it joins the main top and bottom pieces and so goes flat onto the surface where the box is opened. The front-edge of the case often contains a clasp for closure.
In the design of the container, one of its sides has flexible joints that connect it to the main upper and lower parts, permitting it to rest flat on the surface when the lid is lifted. Usually, the front edge of the box features a mechanism for fastening the cover.