The Definition of a Tonneau Cover
The term "tonneau", simply pronounced "ta-no", has its origin from a French word meaning cover, cask, rear sitting compartment of a vehicle.
The term didn't originate from pick-up truck bed covers, but older cars that had an open rear seat in the early part of the 20th century. So this term was originally applied to define the cover of the open rear seat on many of these early automobiles.
Here is a photo of an early 20th Century car with an open rear seat that could use a 'tonneau cover'
The Function of a Tonneau Cover
Generally, tonneau covers (also referred to as truck bed covers) are made to perform these functions:
"Out of sight...out of mind" Tonneau covers assist in keeping your personal items such as tools, equipment, groceries, and anything else that you might place in your truck bed from the eyes of the public and potential thieves.
Protecting your personal items from extreme weather conditions such as snow, rain, dust/dirt, and the sun. Tonneau covers also protect the items in your truck bed from blowing out as your driving down the road.
Getting better gas mileage with your tonneau cover is another benefit that's become more popular over the last few years. Some tonneau cover manufacturers claim an average savings of around 6% to 8%. Some claim more...some claim less...some make no claim. If saving $$$ on gas is your greatest concern, then you might want to consider a less expensive tonneau cover to save you money all around.