We are asked this question all the time. The 5th piece basically is to either:
*cover the hump in a rear wheel drive car
-or-
*to protect the rear floor and fill the gap between the two rear car mats
In the U.S., Australia and Europe for the most part, only 4 piece car mats are sold. However, in Asia, car owners expect the 5th piece. A few car mat companies specifically Posh Pile, and Weathertech offer the 5th piece and car owners seem to appreciate this extra protection. This photo is a Posh Pile carpet mat in a 2011 Volvo XC90.
