Shoemaker 950-18X18 Adjustable Diffuser 18" X 18'' Designed to fit 18" x 18" hole, overall size is 20-1/4" Width x 20" Height. Adjustable aluminum airfoil blades with multi-shutter damper. Vertical air deflectors with nylon bushings for easy adjustment and quiet settings. Countersunk mounting holes for flush appearance with color matching Phillips posi-drive screws. All componet parts, rivets, linkage and handle are enclosed in the valve for trouble-free operation.