Determines where your blind or shade will be installed. Inside: A window treatment installed within the window opening(between window jambs). The factory will take the appropriate deductions for an inside mount application. Outside: A window treatment installed on or beyond the window casing. The factory will not take any deductions. For an outside mount you need 2" of mounting space, thus, the width and height should be a total of 4" or more greater than your window recess.