Shoemaker 425 Series 20" x 20" White Step Down 4-Way Diffuser with Damper (For 20" x 20" Hole) Modern Step-Down Design Actual overall outside dimensions measure 24-1/8" x 24-1/8."Steel construction; for evaporative coolers or central blower systems.Modern step-down design with 4-way air diffusion.Countersunk mounting holes for flush appearance with color matching screws. 4-Way Air Diffusion 4-Way Air Diffusion .