Truck Cargo Box Tonneau Cover System Including A Side-Locking Lock Bar Locking Multiple Slats

A lock bar can be laterally slidable between an unlocked position in which the lock bar is laterally recessed relative to the side rail and a locked position in which the lock bar is laterally extended relative to the side rail. The lock bar can be engageable with, and movable by, the tailgate to slide the lock bar from the unlocked position to the locked position as the tailgate moves from an open tailgate position to a closed tailgate position. Movement of the lock bar between the locked position and the unlocked position can simultaneously interengage and disengage, respectively, with each of the locking surfaces of the plurality of rectangular elements, which interengagement limits upward movement of the rectangular elements.