A Tonneau Cover is a flat cover that can be secured to the top of the truck bed. Like a technologically advanced tarpaulin. A Tonneau protects your truck bed and smaller cargo from the ravages of weather. Most Tonneaus (or is it Tonneaux?) can be removed easily. Tonneau Covers come in several sizes to fit different truck beds and are closed by snaps, a hard cover or a roll cover. Soft Tonneau Covers are made of vinyl, which can shrink in cold weather. Be sure to ask about tension control, some covers have systems that compensate for weather conditions.