Inside Mount shades install within the frame or casement of your window, Measure the width and length of the window opening in three places and choose the smallest measurement. The factory will make your shades with a 3/8in. deduction to the width to ensure a perfect fit. Outside Mount shades are ideal where there is not enough depth to the window casement to install your shades. Also consider OM if you are installing black out shades and do not want the light gap around the edges of an IM shade. Allow at least 2 inches on each side and the bottom of OM shades when you measure, to ensure you cover the window opening and check for any obstructions such as trim or handles around the window. Standard shades come on a 1 1/2in. wide headrail and need 1 3/4in. for a flush inside mount. The minimum depth needed to mount the shade is 3/4in.. Shades with continuous cord loop or movable liners have a 2 1/2in. headrail and 2 3/4in. is needed for a flush inside mount. For narrow window casements, a 1 inch wide headrail is available.