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.