Tonneau (pronounced ta’-no) in modern use describes a bed cover for a pickup truck. It can be hard or soft. Hard covers optionally hinge or fold.
Its original (now archaic) use was for an open rear passenger compartment on an automobile and, by extension, a body style incorporating such a compartment. The word comes originally from French, roughly meaning cask, container, or cover. Most tonneaus were fixed in place as an optional element at purchase, but some could be removed as on the Crestmobile. Early tonneaus had a rear-facing hinged door as a rule, but single- or dual side doors were soon introduced. The first side-door tonneau was made by Peerless, but others quickly followed suit. This led to the development of the modern sedan/saloon, with Cadillac manufacturing the first production closed-body four-door car in 1910.
In the post-World War II era the term has come to refer to the area behind the front seats of an open car (a convertible or roadster). The term tonneau cover is sometimes used for a hard or soft cover that encloses the well for the convertible top and/or the rear seating/storage area. The tonneau cover dates to the days of horse-drawn cargo wagons, and later to early trucks, where a tarpaulin was commonly used to protect a load from road dust or weather. Similarly, early open-bodied touring-type automobiles used tonneau covers to protect unoccupied rear seats. Quite often the word is spelled incorrectly as "tonno".
A tonneau covering the rear half of bed was often used on electric trucks to reduce wind resistance and increase the range.
The most basic type is made from cloth or vinyl and uses a rib-like system to support the fabric and keep it taut. A snap based system can also be used, but is less common now because most truck owners do not want to install the snaps on their vehicle as they typically require drilling or using permanent adhesive.
The next most common type of tonneau cover is called a "roll-up", which is a unit mounted at the front of the bed just behind the cab. This unit is usually a small rectangular box attached at each side of the bed. Inside this box is a retractable fabric or plastic cover that can be rolled out and secured to the end of the bed. These have the benefit of being more convenient to use as they don't involve any difficult or time consuming assembly or disassembly unlike other styles.
Fiberglass or hard plastic tonneau covers are quite popular because they have what is generally agreed upon to be the best appearance. They tend to be painted to match the truck (unlike the other styles which are generally black), they are solid in construction, and they can be locked making them more secure than the other options. These covers are usually heavy and require gas struts to assist in opening and closing. They operate much like a vehicles hood, typically opening from the tailgate end of the bed (back to front). However, some are available with multiple compartments that will open front to back, back to front, side to side, or even raise vertically. Fiberglass or hard plastic tonneau covers are sometimes installed as a factory option with some new vehicles, such as the Chevrolet Avalanche, Ford Sport Trac, and Honda Ridgeline (to name a few).