Round Aluminum Butterfly VentRound Butterfly Vent, Polished Aluminum, 7 3 4" Diameter This butterfly vent is commonly used to increase air circulation in all varieties of trailers. This vent is made of polished aluminum, much better than the old chrome product, because they will never rust. Can be adjusted for a small amount of air, or fully open. It features two triangular holes, 3 1 2" X 2 1 2' X 21 2". Operated from the exterior of the trailer. Installs over a 5 3 4" to 6"