This is for a (Gray) Two Piece Side Wall Air Vent. GET RID OF ALL THE FUMES AND GASES THAT CAN BUILD UP IN YOUR TRAILER!!! This vent is designed to scoop air from the outside by using the wedge shaped outer piece and circulate it through the round inside vent into the trailer. The outside piece is designed to let air in but keep water and other elements out. The installation requires about a 3" diameter round hole. The outside piece measures about 8" by 6" and the flange diameter on the round piece is about 4 1/4". An additional vent can be used on the opposite wall to dramatically increase the amount of airflow. Install the outer piece facing forward on one wall and then on the other side install the outer piece facing rearward). By doing so you will increase the air movement through your cargo area. A Great way to let your trailer breath.