Kompell Aire Return Grille listed size 8” wide and 10” tall is for the duct opening measurement. Surface outside dimension is 9.75” wide and 11.75” tall. Size Selection Guide 1. Measure the size of the air duct opening(the size of the “hole” ). 2. Select the listed size that match the duct opening size. 3. The outside or overall size is 1.75” larger for both width and length so it can cover the “hole”. 4. 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 8" wide by 10" tall, buy a 8x10 grille; If the hole is 10" wide by 8" tall, buy a 10x8 grille.