A usual way to convert an open pick-up truck to a closed one is to mount a cap above the load area and to secure the top of the walls of the load area to the bottom of the walls of the cap.  The cap comprises right and left side walls, a roof connecting the sides, a forward end which overlies the forward end of the load area and a rear end which overlies the rear end of the load area.  The rear end wall of the cap may be a hinged lift panel and the front and rear ends of the cap may be windowed.  The rear end of the load area may be a hinged drop panel. 
The removing and mounting of such a cap is arduous and time consuming and involves the cooperation of at least two people as caps are heavy. 
There have, therefore, been various alternate constructions providing convertibility to pick-up trucks so that the load bed may be covered for camping or protected hauling, for example, and open for access to the bed or to extend the area above the load bed.  Examples of such alternate construction are disclosed in Fletcher U.S. Pat.  No.  3,995,890; Hather U.S. Pat.  No.  4,284,303; Garvent U.S. Pat.  No.  3,858,744; Wilson et al U.S. Pat.  No.  4,324,429 and Jensen U.S. Pat.  No.  4,629,243. 
Jensen teaches the provision of a mountable hinge kit for opening vehicular enclosures and which permits selective access to either side of the truck bed.  The kit must be removed, however, to provide an open truck bed. 
Wilson et al.  attaches a cover to a pick-up bed, which cover is hinged at one of its ends along one end or side of the bed to provide access at the other end of the cover.  The cover is also removed for providing an open truck bed. 
Garvent provides a pair of cover panels extending longitudinally of the bed of a camper, the panels being hinged at their outer edges to the sides of the bed and openable to vertical and outwardly extending horizontal positions.  When closed, the bed is actually not usable, except for hauling gear, as there is no access to the load bed. 
Hather discloses a pick-up truck cover and hinge construction in which side cover sections are hinged to the sides of the bed and which permits access to the load bed beneath the cover from either side or from the rear; and, in addition, provides for vertically extending the sides of the load bed by raising the hinged side cover sections. 
Fletcher discloses a shell disposed in the load bed with side walls extending from the bed floor to a roof which is longitudinally divided, the divided panels of which are moved by control members from closed horizontal position above the load bed to a vertical position proximate the outer surfaces of the side walls.  All of these prior art constructions require custom made cover or shell structures and/or complicated and expensive control members for effecting the conversion.