HBW Return Grille listed size 24” wide and 18” tall is for the duct opening measurement. Overall outside dimension is 25.75” wide and 19.75” tall. Difference Between grilles, registers, diffusers and vents?Grille – a series of deflectors in the shape of blades or fins fixed in a frame to allow air to pass through from duct to room space or from room to outside via ductRegister – a grille attached with a damper or valve to control the air flow volumeDiffuser – a unit with duct connecting piece and a larger face piece with multiple blades to spread out air flow to multiple directions. Examples are drop ceiling T-bar diffusers and round ceiling diffusers.Size Selection GuideMeasure the size of the air duct opening(the size of the “hole” ). Select the listed size that match the duct opening size.The outside or overall size is 1.75” larger for both width and length so it can cover the “hole”.Do not use outside measurement for size selection or it would be too bit to fit.Width vs HeightThe first measurement in a return grille size is the width(horizontal measurement), the second measurement) is the height(vertical measurement). For example: if the hole on the wall is 24" wide by 18" tall, buy a 24 x 18 grille; If the hole is 18" wide by 24" tall, buy a 18 x 24 grille.