a tonneau system for a cargo box of a vehicle including a support frame , a cover spanning the support frame ; and a clamp . the support frame includes side rails having an inboard section and an outboard section , such that the outboard section is positioned adjacent to the top surface of the sidewall of the cargo box to aid in the supporting and positioning of the support frame . the clamp is capable of hanging from the support frame during installation to improve the ease thereof . the clamp also includes a pair of members being positionable in any one of a plurality of discrete positions for improved installation . a front rail adjustment mechanism permits the selective tensioning of the cover to maintain a predetermined load therein . while a front rail retaining mechanism is slidably coupled to the front rail member to prevent the front rail member from being inadvertently disengaged from the side rail members . a rear rail retaining mechanism is used to lock the rear rail member or unlock the rear rail member and urge the rear rail member upward .