It isn’t just where the headrail will be mounted! While the simple answer is, “Inside” fits on the inside of the window opening and “Outside” mounts to the wall on the outside of the window opening, there are other things to consider when choosing which mount type is best for you and for the room they are intended for. Inside: This is the cleanest look and the measurements need to be accurate and precise. The fabric is closest to the window and the product can get caught in a gap (if not measured correctly) that is narrower and can essentially disable your window covering. Outside: This opens up nearly all product options but, it uses more material which can translate into more cost. Also, trim has to be taken into consideration when mounting. Light Gap: Another consideration is the light gap. While both an outside and an inside mount will have a light gap (unless channel is used such as with Hunter Douglas’ “LightLock”), the light that is bleeding into the room can be dramatically different. With Inside mount, the light is bleeding into the room directly. With outside mount the light is cast onto the wall.