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.
One of the sides of the container has flexible links that connect it to the main upper and lower parts, enabling it to rest flat on the surface when the lid is lifted. Typically, the front edge of the box contains a mechanism for fastening the cover.