The blind is mounted inside the window opening for a clean, tailored look. When selecting the ordered width and height, enter the exact dimensions of your window opening, as detailed in the measuring instructions. Do NOT enter the size of an old/existing blind. The factory will make the necessary width deduction for a proper fit. Please make sure your window meets the minimum inside mount depth required to accommodate the brackets, or the flush mount depth if desired. Depth requirements can be found on the product page.